Returns specified value from a data extension. You can specify multiple additional field and value pairs as part of an AND clause.

Lookup(1, 2, 3, 4)

1stringRequiredName of data extension from which to return the specified value
2stringRequiredName of column from which to return a value
3stringRequiredName of column used to identify row containing lookup value
4stringRequiredValue to match string against

Returns the city associated with the postal code 46016 in the PostalCode data extension: Anderson, IN