The description that follows is here so that no one is misled, thinking that Baptists and Catholics are the same. While many things in a church service might seem similar, there are foundational differences in our teaching, our view of God's Word, our view of the Church, and most importantly because of its eternal consequences, our view of salvation as it is taught in Scripture.
Some basic similarities and differences...
We agree on quite a few important things. We agree there is one God, triune in three persons; we agree in the virgin birth of Jesus Christ; we agree that God raised Jesus from the dead; we both believe in the deity of Christ; we believe that the Bible is God's Word; we agree that Christ is returning in judgement of sin; we agree that because of Adam's original sin all humanity is ruined in sinfulness and we need a Saviour. We also agree on the sanctity of life and the holiness of marriage.
Some simple differences are that Catholic churches are led by priests who are addressed as "Father" and remain celibate, while we have churches led by pastors who are addressed as "Pastor" and can get married. It is also good to note that a Baptist pastor does not take confession or prescribe penance or declare absolution for sin. We believe that only God can do these things.
How did the Baptist Church split away from the Catholic Church?
The Baptist Church did not split away from the Catholic Church directly. In 1517 in Germany, a Catholic monk named Martin Luther wrote 95 arguments against the unbiblical practice of selling indulgences (an indulgence is a grant purchased from the Catholic Church to get a soul out of Purgatory sooner).
This brought Luther into conflict with the authorities of the Catholic Church, and through this conflict he realized that the authority for true believers in Jesus Christ comes not from the Church and the Pope but from God's Word alone. This sparked the Reformation, where people called Protestants were protesting against the errors in the teaching and practice of the Catholic Church when measured by the Bible.
The Protestants stood for 5 SOLAS (sola is the Latin word for "only"):
SOLA SCRIPTURA - "by Scripture alone"
SOLA FIDE - "by faith alone"
SOLO CHRISTO - "by Christ alone"
SOLA GRATIA - "by grace alone"
SOLI DEO GLORIA - "for the glory of God alone"
About 100 years after the Reformation, a group of Protestants in England who followed the teachings of the Reformers realized that according to Scripture, baptism was for believers alone, and so they called themselves Baptists. You can read the London Baptist Confession of Faith 1689 for yourself. We are among the descendants of those early Baptists.
Authority
The bottom-line foundational difference between Baptists and Catholics is found when we talk about authority. We believe that the Word of God is the only authority for faith and life. Catholicism adds the authority of the Magisterium, Pope, and Councils, such that the Catholic Church has the power to create teachings not found in Scripture. We believe the Bible has only 66 books; the Catholic Church adds the Apocrypha writings to these books. Catholics often assert that we wouldn't know what books made up Scripture unless the Church had put its seal of approval on them, but this simply isn't true.
The Mediator
We believe there is only one Mediator between God and Man, the Man Jesus Christ (1 Timothy 2:5). No one else can intercede, and no one else can act as middle-man between us and God for salvation. The Roman Catholic Church agrees that Jesus Christ is the only Mediator between God and humanity, but then they put a mediator between the Mediator and humanity. This mediator is the Church/the Pope/the priests. In effect, they are saying that you cannot come to Christ to be saved unless you come through the Catholic Church under the Pope and his priests. And so it happens that many put their faith in the mediator rather than in the Mediator, who has been eclipsed. Priests act as mediators in the sacrifice of the Mass and in confessionals when they pronounce forgiveness of sin, something only God can do. This gives many adherents of the Catholic Church a false security in a false salvation, because they are trusting in the ecclesiastical mediator rather than the only true Mediator.
Salvation by Grace alone through Faith alone in Christ alone
We agree with Catholics that salvation is by grace through faith in Christ. The key issue of essential disagreement is the word "alone", and it makes all the difference in the world. We believe the Bible clearly teaches that all of our best and most righteous works are filthy rags in the sight of God. Therefore we cannot bring any of our good works to Him in order to receive salvation. Instead, salvation is the result of His act of grace enabling us to have faith in the Lord Jesus Christ. The new life that we receive in Christ presses us on to good works, and good works are the evidence of the Holy Spirit's presence in our lives. The Catholic Church teaches that it is by faith and works together that humans can earn the favour of God and so receive salvation. (More precisely in Catholic teaching, good works maintain the justification infused at baptism.) The problem is that good works will always take precedent so that humans will rely on their efforts to earn and achieve salvation rather than on God. In so doing, people are not actually coming to God through the Mediator, but instead they are relying on themselves or upon an institution to save them.
The Lord's Supper vs. Mass
The Lord's Supper (also known as Communion) is one of the two commands the Lord Jesus gave us to observe, along with baptism. We believe in Believer's Baptism by immersion (dunking), which means that a person receives baptism after he/she has put their faith in Jesus and that immersion is the only Biblical manner of baptizing, as it represents dying to sin and rising with Christ to new life in Him. Baptism itself does not save, but it confirms and confesses saving faith in the Lord Jesus Christ. Through baptism we enter into the family of God, and in the Lord's Supper we are continually encouraged and spiritually refreshed. The Lord's Supper is where we remember the death of the Lord on the cross as He commanded us, eating the bread and drinking the cup, testifying to our faith in Him and proclaiming His death until He returns. While we believe that the Lord is present by His Spirit in a special way with His people in the Lord's Supper, we do not believe the ceremony in itself possesses any power to save. The Catholic Mass, on the other hand, is performed by a priestly mediator who invokes the actual body and blood of the heavenly Christ into the elements of bread and wine, replacing the bread and wine. Therefore, when adherents eat the bread and the priest drinks the wine, the actual flesh and blood of Christ is being consumed, as if Jesus is being sacrificed all over again. We believe that the sacrifice of Christ happened only once, on the cross of Calvary, and this was a once-for-all sacrifice. Therefore we completely reject the Mass as an erroneous practice and the expression of a false gospel, because Christ's sacrifice on the cross is not a finished work but rather a perpetuated bloodless sacrifice that must be maintained by the priesthood, an entirely unscriptural idea.
The Virgin Mary
We believe according to the Bible that Mary was specially chosen to bear the Messiah and should be honoured as a heroic woman of faith. We believe in the virgin birth, that Jesus was specially conceived by the Holy Spirit (in no carnal sense). But we do not believe that Mary was preserved from Adam's original sin so that no taint would come to Jesus. Mary was a sinner in need of a Saviour just like the rest of us, and she was no longer a virgin soon after Jesus' birth, for she bore at least 3 other sons. Therefore we do not adore her with lofty titles such as "Queen of Heaven", "Mother of God" or "Our Lady of the Angels". She should not be prayed to, and she has no power to help us or to save. We believe that the dangerous Mariology of Catholic teaching can easily become blasphemous in Catholic practice.
Praying to Saints
We do not believe in a hierarchy of Christians, where there are regular believers and then the Saints. According to the Bible, all true believers in Jesus Christ are part of the family of God, and so all true believers in Christ can be called "holy ones" or "saints". Because we consider prayer a part of worship, we believe all prayer must be directed to the triune God alone. We consider it improper and blasphemous for prayer to be directed to a saint when it should be given directly to God alone. If a saint answered someone's prayer, then that saint would receive the gratitude and the glory. God does not share His glory with anyone.
Icons & Images
We believe that prayer or worship directed toward any image of Christ or the "Saints" breaks the 2nd Commandment and is therefore an act of idolatry. Therefore you will not see a crucifix in any Baptist Church. We believe that the glorious message of the Gospel is that the cross is empty as well as the tomb. Jesus does not need to be re-sacrificed over and over again. He has once and for all triumphed over the cross, sin, death, and the grave and has been exalted to the right hand of the Father.
Purgatory
We do not believe that the Bible teaches at all anything even remotely close to the Catholic teaching of purgatory, where anyone who is less than a Saint goes after death to be purged of remaining unrighteousness in order to be purified enough to enter heaven. The Bible, on the other hand, clearly teaches that on the Day of Judgement those who have trusted in Christ for salvation will receive eternal Life, and those who have rejected Christ will receive eternal punishment.