First of all, please note that Roman Catholics are Christians so you can't really say there's a difference between Roman Catholics and Christians. Perhaps you can say "Protestants" or "other Christians."
There are lots of differences but perhaps the main one is that Catholics believe more than most Protestants in a person's ability to get to heaven based on actions (doing what the Church says) rather than on faith.
Catholics are also much more ritual-oriented than most Protestants and give more of a role to saints and to the Virgin Mary.
Catholics go to Mass in order to receive Communion. (There's another difference -- Catholics believe the bread and wine literally become the body and blood of Christ). They also go because the Church says you must. Finally, they go for the same reason others go to Church -- to commune with God.