As everyone has noted, I'd like to summarize:
Venial are sins that are "small" sins, so to speak. We know that there are multiple levels of sin, because the Bible speaks of it, right?
So venial are like what Shelb said, being impatient, getting a little angry with people because you're a little too impatient...but you don't go as far as actually jumping on people's cases...
Mortal sin is like Shelb said, i.e. abortion, you tell your sister off because she just ripped your sweater, you gave consent and you knew to do good and you didn't do it and so thus you sinned...
The thing to remember, is not get wrapped up in all of the, "Oh, well, is this a venial? So is this a mortal? Oh no, if this is a mortal, and I don't think it is, and I don't confess, am I going to hell?", and then before you know it Satan has you all in a big tangled mess and you just give up and you live by the, "all sin is the same no matter what" notion that most Protestant churches teach.
The real goal here is for you to listen to Christ speak in your life and to grow stronger as you now notice those little things that you need to work on...that's the main difference with converts and this whole topic of sin. You have to start realizing that you sin a lot...a lot is venial, and you might not even realize you did, depending on how strong you are with Christ anyway...
You'll know, as you grow in Him what you're supposed to do. You'll know that standing in line at the grocery store tapping your foot so to speak waiting for that little old lady to hurry up with the little "pocketbook" full of change, is probably a show of impatience. And you'll probably catch yourself and think, "Ooops, maybe I should work on my impatience..." (venial)
Versus
Someone flips you off as they cut you off on the road, and instead of smiling and laughing at them, you return the favor and decide to cut him off and you flip him off as well. (mortal)
The whole key here is growing in Christ. As we recognize our faults, we should partake in the Sacrament of Reconciliation as a means to draw us closer to God by receiving his sanctifying grace -- which we only receive in the Sacraments...(help me here Shelb if I'm wrong with sanctifying vs. non-sanctifying)
We don't have to confess our venial sins, or as I call them, our imperfections, but the Church does encourage us to do so...
Well why? Because confessing our venial sins allow us to grow stronger. I think, if we can reduce our venial sins in our life, then we will be purer in God's eyes...and we won't
want to sin and commit mortal sin...we will be stronger not to...
And that's the whole purpose to confession...to draw us closer to God -- not to make us feel horrible...God is a God of love...