Integrated Circuits is a mod that allows you to combine the logic gates that were first introduced by RedPower, and have been adapted several times since, to assemble them into a single block.
Circuits can have three different input modes: analog, digital and bundled. For the bundled input, a third party mod is recommended that adds bundled wires (however, they can interconnect with each other without the need of a wire in between). A full list of mods supported can be found at the end.
The second main feature are 7-Segment displays which are multi-blocks that can display a signal in a human readable format, such as 16 bit shorts, floating points or a hex character representing analog input.
The main purpose of Integrated Circuits is to serve as a tool in complicated Redstone contraptions or to control complicated machinery without giving up the concept of Redstone and retaining the feel of building the circuit yourself. It is also a lot faster than vanilla Redstone, compact, and replicating circuit designs is easy as they are saved in the form of blueprints which are interchangeable.