The hibernation mode (also known as Suspend-to-disk) is one of three power-saving modes for the ThinkPads. In this mode, all tasks are stopped and memory data and current system status are stored on the hard-disk drive. This feature requires the presence of a hibernation file on the hard-disk drive. You can enter this mode by pressing. When in hibernation mode, all indicator lights are turned off. To resume operation, turn the power switch.
See How to make use of Power Management features to learn about getting hibernation mode and other power-saving modes to function on a Thinkpad running Linux.
Beside the BIOS-controlled hibernation, for Linux there also exist two software implementations of that functionality: swsusp (included in recent Linux kernels) and TuxOnIce (currently requires a kernel patch). Windows XP also has a software implementation of this capability. Each implementation has different requirements and features.