When a computer hibernates, it saves RAM contents to a swap file or partition and completely powers off. Another operating system can be booted if necessary during the time the other system is hibernated (however, this is not recommended). When the computer is resumed from hibernation, it copies this data back into RAM and resumes with the same state and opened programs as before.
Hibernation on Ubuntu is initiated by executing
pm-hibernate with root privileges. Because it does not resume properly on some computers, possibly corrupting opened filesystems or files and destroying non-saved data, hibernating is not available by default in the main power menu at the upper right hand corner in Precise or Quantal but must instead be enabled manually.
Touching a filesystem (or swap partition) in use by a hibernated operating system is dangerous, and so Ubuntu will not mount a hibernated Windows drive. It must be either mounted read-only (preferred) or have its hibernation file removed before it can be accessed.