I have often got this question on this blog as well in different SQL Training.
What is the difference between Update Lock and Exclusive Lock?
When Exclusive Lock is on any processes no other lock can be placed on that row or table. Every other process have to wait till Exclusive Lock is complete its tasks.
Update Lock is kind of Exclusive Lock except it can be placed on the row which already have Shared Lock on it. Update Lock reads the data of row which has Shared Lock, as soon as Update Lock is ready to change the data it converts itself to Exclusive Lock.
In some other posts, we will discuss locks in more details. Let me know if my above explanation is clear enough.
Reference: Pinal Dave (http://blog.SQLAuthority.com)