oh-my-zsh is an addon to zsh that adds additional functions, autocomplete helpers and themes.
oh-my-zsh is an extension to zsh that adds additional functions, autocomplete helpers, syntax highlighting plugins, and themes. It's hosted on github and is described as:
A community-driven framework for managing your zsh configuration. Includes 40+ optional plugins (rails, git, OSX, hub, capistrano, brew, ant, macports, etc), over 80 terminal themes to spice up your morning, and an auto-update tool so that makes it easy to keep up with the latest updates from the community