SQL Server Isolation Level

Read Committed

Read UnCommitted

Repeatable Read

Seriazable

Snapshot

Read Committed

READ Comitted  take only Committed value if any transaction pending or incomplete wait for till operation complete

Read Uncommitted

If any table is (updated Insert delete update) and transaction is waiting for complete or roll back  uncomitted  value display  it is called dirty read

if we want to read only committed rows use  keyword with(nolock)

  select * from Emp with(nolock)

http://www.besttechtools.com/articles/article/sql-server-isolation-levels-by-example