Screenshots pretty much explain what it does. If you want to get a feel for how it behaves, check out the CodeMirror demo's.
Aside from providing Syntax highlighting it has the following features:
- Zen Code Completion
- Format code
- Emacs / VIM keymaps
- Themes
- "Save Template" keybinding
- Maximized mode (fullscreen)
Credits for the editor go to CodeMirror, credits for Zen Coding go to Sergey Chikuyonok.