Hidden Protected Area
The Hidden Protected Area is a special area on your harddisk that can be hidden to the software running on your ThinkPad. It includes all the software and data needed to recover the preloaded state of the ThinkPad. The HPA also includes some diagnostic tools and a (MS Windows only) backup tool. The Hidden Protected Area was introduced with the R/T/X 40 series of ThinkPads and is present on the preinstalled harddisks of all ThinkPads shipped by IBM since then.
Rescue and Recovery
Recent ThinkPads are shipped with the Rescue and Recovery technology. That technology uses a (hidden) partition for recovery of the preloaded state and diagnostic tools.