Codevision AVR C and Atmel AVR C compiler
There isn't really much difference there may be some differences in which chips are supported under Codevision so you may want to check for the chip you plan to use. 

There are different IDE's available Codevsion has its own IDE and there are different IDE to choose from for Atmel's avr C (like AVR Studio 4) But you could also use WinAVR which is uses GCC, or you could use GCC with another IDE of your choice or you can go without the IDE entirely. If you do use GCC this is a good website to check out AVR Libc. For even more information and tutorials about AVR you should check AVR Freaks.