I hate being legalistic ;), but I think this would be a good analogy.
In criminal law, most crimes consist of two elements:
mens rea - A mental state.
actus reus - An act.
So we commit a crime, when we are on a mental state and we act on it. The severity of the crime is dependent on the degree of the mental state and the degree of the act.
A mortal sin, is a sin *done *in full *knowledge *of it being a grievous offense against God.
The CCC says about sin:
1850 Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight."122 Sin sets itself against God’s love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become "like gods,"123 knowing and determining good and evil. Sin is thus "love of oneself even to contempt of God."124 In this proud self- exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.125
A mortal sin is what St John talks about in 1 John 5:16 If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that. 17 All wrongdoing is sin, but there is sin which is not mortal..
Further, the CCC says:
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."131