With a tracker, its easier to manage your poker games when you'r multi-tabling. It also allows you review your played games faster and with better accuracy than doing it manual.
For poker software choice, i suggest checking the stickys in CC "Poker Software & Tools" page.
"PokerTracker is one very popular software, although I don't use it myself. They have a free trial period.
But you should be aware that all poker sites doesn't look too kindly towards people who use these HUDs, so always read poker site's TOS carefully before taking any action, and If necessary ask from their support."
If you are serious about poker, you will eventually want to use some kind of software to help you get better. Poker Software is much more than trackers. It is also equity calculators, solvers, AI programs, ICM programs etc.