Lent starts today, Ash Wednesday, Wednesday, Feb. 22 and goes through Saturday, April 7.
Here is a great article about the History of Lent and another on the Season of Lent. And to round it out, here's the wikipedia link on Lent.
The act of giving something up for Lent is symbolic to preparing for Christ's sacrifice. When you give something up, it's supposed to be meaningful and difficult to remind you of Christ's difficult, deadly sacrifice.
In the past, I've given up coffee, sweets, swearing, and eating out. It's never easy.
This year, I'm giving up sweets again. I'm a huge sugar junkie. Sacrificing my sweets will continually remind me of Jesus' sacrifice for me.