Time Machine does exclude some things automatically, such as system work files, most caches and logs, your trash, etc. That's usually a few GBs, though, not 100!
If the backup completed normally, then it backed-up everything it needed to. It sounds like you have something extra-large (and somewhat hidden) taking up a lot of space. See Where did my Disk Space go? for some common culprits.
For other info on Time Machine, you might want to start with the Time Machine Tutorial, and perhaps browse Time Machine - Frequently Asked Questions. Questions #14-17 and 19 there cover the various ways to see and restore from your backups. That's a lot of not-so-light reading, so don't hesitate to post back with any other questions.
By the way, Time Machine is the software that performs backups, to various places. A Time Capsule is a piece of Apple hardware combining a wireless router and hard drive, and is a good way to make wireless backups with Time Machine.