As you can see, read uncommitted is the least restrictive isolation level,
and serializable is the most restrictive.
The default SQL Server isolation level is read committed.
As the level of isolation increases, SQL Server holds more restrictive locks
and holds locks for longer periods of time.
Since the isolation level affects the locking behavior for SELECT statements,
isolation affects the locking mode used on data that is being read.
Source:
Microsoft SQL 2005 Administrator's Companion,
Chapter 17 - Transaction and Locking
Page 487