info on .m4a and converting to mp3

Hello,
I have just realized that when taking some songs off of my itunes, the songs are now .m4a
Is this an itunes format?
I have been putting some music up on my site and realized that in one of my older browsers, the .m4a would not play.
What does this file play with? Should I convert it to mp3 and how would I convert this file to mp3.

m4a is part of the digital music formatting battle...it is what(i believe) apple has adopted to use

it can play in WMP with a plugin (do a google search)...i dont believe you can convert it at the current time though
-chris