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deadlock is where transaction A is waiting for transaction B; and transaction B is waiting on transaction A.

Suppose you have 2 threads and infinite history of executions of instructions of 1 and 2 thread. In both threads there is a critical section. Consider the following algorithm:
loop forever
NCS()
want[id] <- 1
loop while (want[1 - id] == 1)
CS()
want[id] <- 0
end loop
// id is either 0 or 1
If there exists a possibility that this algorithm will never enter a critical section of any thread then it is in deadlock. It doesn't seem for the above algo that it may be in the deadlock but indeed it is possible :) It up to you to find why...

deadlock is a situation in synchronous processing , where two are more threads blocked for resources, which are locked by other threads.

4 conditions for deadlock :
a) no pre-emption
b)mutually exclusive
c)hold and wait
d) circular hold and wait

happens when there are 2 threads and each has a lock on a resource needed by other thread (circular hold)

Deadlock is a condition where two or more processes are each waiting for another to release the resource or more then 2 processes waiting for resource in a circular chain.