NOLOCK : Is equivalent to READ UNCOMMITTED (source : MSDN)
NOLOCK or READ UNCOMMITTED Specifies that dirty reads are allowed. No shared locks are issued to prevent other transactions from modifying data read by the current transaction, and exclusive locks set by other transactions do not block the current transaction from reading the locked data. Allowing dirty reads can cause higher concurrency, but at the cost of reading data modifications that then are rolled back by other transactions
READ UNCOMMITTED and NOLOCK hints apply only to data locks. All queries, including those with READ UNCOMMITTED and NOLOCK hints, acquire Sch-S (schema stability) locks during compilation and execution. Because of this, queries are blocked when a concurrent transaction holds a Sch-M (schema modification) lock on the table