Short answer: mortal sin is a serious sin which cuts you off from God. It must contain three elements to be mortal: serious content, personal knowledge of the seriousness of the content, and a willful decision to do it anyway. If any of these three is lacking, the sin is not mortal.
Venial sin is a less serious sin that strains your relationship with God but does not sever it.
As an illustrative exercise (admittedly hyperbolic, but just for the sake of explanation), let's say you decide to sleep with your next-door neighbor's 19 year old daughter, who is engaged to be married. You know you shouldn't do it, and you know it's wrong, but you do it anyway. Is this a mortal sin? Yes.
On the other hand, let's say that you're mentally-deficient, and you don't know the difference one way or the other, and
she seduces
you. Is this mortal sin? No, because there is an impediment that removes the stipulation of complete knowledge of the sin, which renders the sin venial.
Or let's say that Osama bin Laden shows up with a Russian submachine gun, holds it to your head and forces you to sleep with the girl. Is this a mortal sin? No, because the stipulation of complete assent of the will is missing, in that you were forced, also rendering the sin venial.
Or let's say that you decide it wouldn't be right to actually sleep with her, so all you do is stand at the window and watch her sunbathing in the back yard, entertaining naughty thoughts. You try to shake the thoughts off, but they keep coming back. Is this a mortal sin? No, because simply entertaining naughty thoughts isn't enough to constitute a mortal sin; the serious content of the sin itself is lacking.
So that's how mortal and venial sins differ. 1 John 5:16 stipulates that there are some sins that lead to death, and other sins that do not.
Catholics do not believe in "once saved, always saved". We believe you can screw up if you commit a mortal sin deliberately, and if you die with that mortal sin unconfessed, you will go to hell, do not pass Go, do not collect $200.00. Venial sin will not send you to hell, but it can make your faith walk more difficult.
Venial sin is disposed of by contrition, confession, and Mass; mortal sin can only be disposed of by sacramental confession, or "perfect contrition", which we won't bother with for the nonce. Ask me later if you're still curious.
Suicide is treated by the Church as a sin, but whether or not it was mortal at the moment of the act cannot be known, since the individual can no longer be asked why he did it. The Church gives the suicide the benefit of the doubt; since no one in their right mind would willingly kill themselves, the Church assumes that anyone who does was not in their right mind when they did the act, thus removing the complete assent of the will, and rendering the sin venial. After that, all that can be done is to leave the individual up to the mercy of God.
