Time Machine can back up FROM:
Any internal or directly-connected drive/partition formatted with any variation of Mac OSX Extended (HFS+).
In most cases, by default it will:
•Include all internal HFS+ disks/partitions (including externals connected via eSATA)
•Exclude all external disks/partitions, including the one it's backing-up to (but not those connected via eSATA)
You can easily modify the handling of individual HFS+ volumes. See question #10.
It can easily back up multiple Macs. See question #4 for details.
It can easily back up multiple drives/partitions on a single Mac. See question #32 for details.
It will back-up your entire system (OSX, configuration, applications, user data, settings, preferences, etc. (less most system work files, caches, logs, trash, etc.), unless you specifically exclude things (see question #10 and/or question #11).
It cannot back up a Boot Camp or other Windows-formatted volume. The free WinClone app may work for that, or check the Apple Discussions Boot Camp forum.
It cannot back up any network drive, including a Time Capsule, a USB drive connected to a Time Capsule or Airport Extreme, or a Network Attached Storage (NAS) device. The CarbonCopyCloner (donationware) or Chronosync apps (about $40 U.S.) may be alternatives.