TempDB can hold a variety of information.

Work Tables for Groupby, order by, and union queries

Work Tables for Cusor and spool operations

Work Tables for creating/rebuilding indexes that specify the "sort_in_tempdb" option

Work Tables for hash operations

it also contains the Version Store which manages for the instance

Online Index Craion, online index rebuilds

Transactions runnin under snapshot isolation level

Multiple Active Record Sets

...and I'm sure there are a couple things i've missed

http://technet.microsoft.com/en-us/library/cc966545.aspx

How to shrink the TempDB

as you can read below the dbcc shrinkdatabase (tempdb, 'target percent') command only works if there is no activity against the tempdb while the statement is executed

http://support.microsoft.com/kb/307487