Wow... Thats a very general question...
Well, I'm pretty sure there's tons of people here who are willing to write essay answers to them, so I'll just answer the lesser one.
There're 2 seasons of 'Mahou Shoujo Lyrical Nanoha'. The second season is call ' Mahou Shoujo Lyrical Nanoha
A's ' A third one is about to be air on the 3rd(?) of next month call 'Mahou Shoujo Lyrical Nanoha
StrikerS '.
If you want the synopsis for them, I'll direct you to ANN...
http://www.animenewsnetwork.com/ency...me.php?id=4576
My personal opinion, watch it, seat through the painful first few episodes, then you'll see why we have a thread that contains 2000+ posts...
...Oh yeah, ignore the transformation scene if you're squeamish.
, The music's great though.