A deadlock occurs when there is a cyclic dependency between 
two or more threads for some set of resources.

Consider there are transcations going on 1 row, some one is 
trying to update a column in that row while others are 
trying to retrive data of same row,while update locks the 
row for some period of time others cannot select it, hence 
deadlock occurs.

To kill the process is solution. In sql server 2005 use sql 
profiler to view the process, in that see the column named 
transactions if its value is more than 1 right click and 
kill the process.

P1 has lock on R1
P2 has lock on R2
P1 requests lock on R2
P2 Requests lock on R1

Where P & R are corresponding processes and resources.

Deadlock consists of two processes requesting locks on
resources thus, blocking each other.

You can avoid Deadlocks:
Access objects in the same order
Use lower isolation levels
Keep Transactions short and in one batch
Avoid user interactions in transaction
Use bound connections