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More in Deadlocks
What is a deadlock in SQL Server, and why does it happen? How does SQL Server decide which transaction becomes the deadlock victim? What error does an application receive when its transaction is chosen as a deadlock victim, and how should the application respond? How do you capture detailed information about deadlocks that are happening on a server so you can investigate their root cause? How would you read a deadlock graph to identify the exact queries and resources that caused a specific deadlock? What design changes can you make to application code and database access patterns to reduce the frequency of deadlocks?
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SQL Server › Deadlocks

What is the difference between a deadlock and normal blocking, and how do you tell them apart when troubleshooting a slow application?

Intermediate

Answer

Blocking happens when one transaction simply has to wait for another to release a lock, which resolves naturally once the first transaction finishes, while a deadlock is a genuine cycle where neither transaction can ever finish without one of them being forcibly rolled back, meaning blocking causes delay while a deadlock causes an actual error that the application must handle.
-- Blocking: Transaction 2 waits, then proceeds once Transaction 1 commits
-- Deadlock: Transaction 1 and 2 wait on each other, so SQL Server cancels one
Real-world example A support team initially assumes a slow report is caused by a deadlock, but after checking sys.dm_exec_requests finds it is actually just normal blocking behind a long running transaction that eventually completes on its own.

Common follow-ups: How do you check whether a currently slow query is experiencing blocking rather than an actual deadlock?;Does blocking ever escalate into an actual deadlock?

Isolation & Locking;Query Optimization & Plans
All 7 Deadlocks questions