What is the difference between mortal sin and venial sin?

Mortal sins are grave offenses that sever our friendship with God and, consequently, cause a loss of sanctifying grace. Anyone dying in a state of mortal sin would suffer eternal separation from God in Hell.  Venial sins are lesser offenses that injure but do not destroy one's relationship with God.

The gravity of each sin depends upon the nature of the sin itself and circumstances that may either increase or mitigate guilt for that sin. (Cf. CCC 1853-1854) 

A mortal sin cannot be committed "accidentally"; willful or pretended ignorance of divine law does not excuse us and may in fact compound our guilt.  A mortal sin requires three conditions:

• The evil act must constitute "grave matter," a serious offense against God's law.

• The sinner must be fully aware that the act is indeed evil.

• The sinner must fully and freely consent to the evil act, even if he or she does not explicitly and directly wish to offend God. (Cf. CCC 1857-1860)

If any of these requirements is not met, then the sin is considered a venial sin. The sinner's relationship with God and the charity within his or her heart is weakened by venial sin but is not severed. (Cf. CCC 1862-1863)

Mortal sin results in the loss of sanctifying grace and thus requires recourse to the Sacrament of Penance and Reconciliation before any of the other Sacraments may be received, especially the Eucharist in Holy Communion. Venial sins do not preclude the reception of Holy Communion. Nevertheless, the Church recommends frequent confession of venial sins to receive the sacramental grace that gives us the strength to overcome imperfections and habitual venial sins.  It is important to bear in mind that venial sins can easily lead to mortal sin unless they are held in check. (Cf. CCC 1385,1415,1457-1458,1861)

-The Didache Bible