mytheme.styles.css
This is the main CSS file for your theme. Most Sass you write gets compiled into this one long CSS file.

mytheme.no-query.css
This is a "no-query" fallback version of your styles.css that targets devices that cannot read @media queries. It is linked to the breakpoint extension, and it is automatically generated based on your breakpoint settings. Breakpoint also allows you to write additional Sass code that specifically targets fallback or no-query cases. See:
https://github.com/Team-Sass/breakpoint/wiki/No-Query-Fallbacks

mytheme.normalize.css
This is just a copy of whatever normalize.css file you are using, and you probably shouldn't ever edit this directly.

mytheme.hacks.css
This is a special CSS file that is read AFTER all the rest of your theme's CSS is read. It is there so you can add hot fixes or make changes directly in CSS without having to spin up your Omega dev environment and re-compile your CSS again. If you have clean, well-written, complete Sass files and you don't need any weird CSS hacks, then this file should be empty.