What is a NOLOCK?

From: http://www.allinterview.com/showanswers/5546.html

Advantage:
applying nolock in select statement will increase
concurreny and performance in fetching.

Disadvantage:
it does'nt consider the transaction scenario, even if the
transaction is under process not yet commited or rollback
it will fetch the current record
eg

begin tran
insert into tablename
values('1','asdf')

select * from tablename with (nolock)

-- it will display the table with inserted record.
but the transaction is not completed, it can be rollback
so it became dirty read.