In traditional Catholic theology, there is a distinction between mortal sins on the one hand and venial sins on the other. According to the Catholic Catechism, sin "is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods". In other words, sin happens when we do not love God like we should.
Sins can be either venial or mortal. Venial sins are sins which, by failing to love God properly, wound a person's relationship with God but do not destroy it. Mortal sins, on the other hand, are sins which turn a person away from God and destroy their relationship. A venial sin is like picking up your girlfriend 15 minutes late for a date - she'll be angry, but she's still your girlfriend. A mortal sin is like cheating on your girlfriend with another woman - if she finds out, she'll probably break up with you. Or, more accurately, you've essentially stopped being her boyfriend by doing exactly what boyfriends don't do.
Catholic theology does not hold itself to "Scripture alone" like Protestant theology. Catholicism, as explained in the Second Vatican Council, holds that God has revealed himself fully to human beings in the person of Jesus Christ, and that the revelation of Jesus Christ flows to us, as though two streams from one source, through Scripture and the Tradition of the Church.
However, the distinction between mortal and venial sins is to be found in an unsystematic way in Scripture. A distinction is clearly presupposed in 1 John 5:16-17, which the person before me has already posted.
It's all there - "there is sin that is mortal" (16) and "there is sin that is not mortal" (17). Clearly mortal sin is here believed to somehow put the sinner beyond the normal reach of the forgiving action of the church. Venial sin, however, does not. Clearly this is not something which the Catholic Church has just whipped up out of nowhere, as the previous poster pointed out.
As for hell. Well, yes, technically speaking, hell is the eternal resting place for people who have no relationship with God. So if a mortal sin destroys one's relationship with God, then if one dies in a state of mortal sin, one's natural destiny is hell. But theology is not like computer programming, so we can't just identify some conditions under which the exact same thing will happen every time. God's mercy is boundless, and God is free to save whomever God wants to save. It is for this reason that we can never say for certain whether a given person has gone to Hell. This emphasis on God's mercy over God's judgment is found in the Catechism, which, for instance, says that we can and should hope for the salvation of suicides (even though murdering yourself is perfect matter for being a mortal sin).
For that matter, not all really bad acts are mortal sins. For something to be a mortal sin, it must be (1) objectively grave, that is, actually a bad thing; (2) done in full knowledge; and (3) done with the full consent of your will. If you accidentally killed someone, it wouldn't be a mortal sin because you didn't do it with full consent of your will. If you told your teacher that your dog ate your homework it wouldn't be a mortal sin, because what you did is not objectively grave. If you cheated on your husband because it was dark and you thought his brother was him, you wouldn't have done it with full knowledge - therefore, no mortal sin.
The ordinary means for being freed from mortal sins is the Sacrament of Reconciliation (or Confession), in which a priest, as the representative of Christ, forgives the sinner in the name of Jesus and reconciles them to God and to the Church at the same time. It's like if you'd cheated on your girlfriend - you'd need to formally apologize before she took you back.
The Sacrament of Reconciliation is a celebration of the boundless mercy of our God who is love. The Sacrament ends with one of the most beautiful prayers I know:
"God, the Father of mercies,
through the death and the resurrection of his Son
has reconciled the world to himself
and sent the Holy Spirit among us
for the forgiveness of sins;
through the ministry of the Church
may God give you pardon and peace,
and I absolve you from your sins in the name of the Father, and of the Son and of the Holy Spirit."