If two or more transaction is accessing the same resource at same time. There is a problem of concurrency. Lets discuss what are the common problem arises in SQL Server. 1) Dirty Read 2) Lost Update 3) Non-repeatable Reads 4) Phantom Reads
Here is the pictorial representation to show , how the dirty read occurs in the system. This is mainly occurs when we set the Isolation level from Read Committed to Read UnCommitted(NoLock). There is no difference between Nolock and Read UnCommitted in SQL Server.