Theos is a cross-platform suite of development tools for managing, developing, and deploying iOS software without the use of Xcode.
The Theos suite of tools consists of a handful of important components:
- A project templating system (NIC.pl), which creates ready-to-build empty projects for varying purposes
- A robust build system driven by GNU Make, capable of directly creating .deb packages for distribution in Cydia
- Logos, a built-in preprocessor-based library of directives designed to make MobileSubstrate extension development easy
Theos is primarily used for jailbreak-centric iOS development (such as MobileSubstrate extensions, PreferenceLoader bundles, and applications intended for distribution in Cydia), but can be used for other types of projects as well. This can be helpful for someone wishing to develop an iPhone SDK-based application without using Mac OS X or Xcode to do so, as Theos can be used on Linux and iOS as well.
Theos also includes Logify, which accepts a class header and generates a MobileSubstrate extension which hooks all of that class's methods, printing log messages when they are called. This helps a hook developer see when certain methods are invoked during use.
This Stack Overflow answer may also be helpful.
For reference, check out this Theos documentation by theiostream.