(NOLOCK) allows dirty reads. Basically, there is a chance that if you are reading data out of the table while it is in the process of being updated, you could read the wrong data. You can also read data that has been modified by transactions that have not been committed yet as well as a slew of other problems.

Best practice is not to use NOLOCK unless you are reading from tables that really don't change (such as a table containing states) or from a data warehouse type DB that is not constantly updated.