Answer:
A venial is a slight offense against God while a mortal sin is a great offense. A venial sin is like a spiritual misdemeanor while a mortal sin is like a spiritual felony. Things like telling lies to get out of trouble, stealing something that is not of great value, or profanity are examples of venial sins. Murder (including abortion), stealing something of great value, and not going to Church on Sunday (which violates the Third Commandment) are examples of mortal sins. Mortal sins are so serious that they drive sanctifying grace from the soul. If you die without having been forgiven for a mortal sin, you will go to hell. Venial sins only lessen grace. No amount of venial sin is enough to send someone to hell. If you die without having been forgiven for venial sins, you will have to spend time in purgatory to have them forgiven.
Really though, God looks at all sin the same. Read Rev. 21:8. Also, one scripture says why should we keep on sinning, is it so that grace may increase. If you don't repent from any kind of sin you will go to hell. If God sees the effort, you are fine because he understands nobody is perfect and we all mess up.