I bought my first baitcaster this week and had a question.  What exactly does the braking system do and how do I use it?

Here's a link to the reel I bought:

http://www.basspro.com/webapp/wcs/stores/servlet/Product_10151_-1_10001_90567_100001001_100000000_100001000_100-1-1

The braking system is what controls that little spindle that holds the line.  Basically it keeps the spindle from spinning too fast, or too slow.  If you have it adjusted properly, you should have a smooth cast every time.  Until you change your lure at least...  Then you should recalibrate it to match the weight!  

I've mostly been using the cast adjuster for that.  It seems kind of redundant.  What's the difference between the two?

One is for low speed and one is for high speed..... high speed =cast control knob       low speed =magnetic  .The control knob on the right makes it so your lure don't cast as fast at the start which is the time when your spool spins fastest the startup.  The other control is as ur lure slows down the magnets stop the spool on the other side from still spinning at slow speed which is the end of your cast   GOOD LUCK

i got the 7;1 sweet reel never had a backlash yet, for light lures i only have 3 brakes on, very easy to use but it did take a little bit for me to be accurate with it, now i usually come within 6- 12'' of a target, practice,practice