TPM

The TPM is an required part for loads of the macs in use today. It is required for Mac OS X to even load, as a security measure to make sure you're running it on official Apple hardware. Many groups do not like this technology as it is closed and horrible, yet it claims to only do security operations.

Technical Information
While we can't tell you how to use it, here's some basic facts about the part. It's basically there to do security operations (encrypting, decrypting etc) very quickly and easily. These modules are only generally used by Microsoft, Intel, HP, Apple and AMD.

Apple uses this for it's DRM and Mac OS X licensing as it is soldered onto the motherboard, it is registered to that particular machine.