These work in the same way as numeric comparisons (a date is just a number, starting at 0 for 1/1/1900 - see Numeric comparisons), except you should enclose the dates in a pair of #s.
For example, using the Employee table, you can find out who has joined since 1990:
SELECT LastName, JoinDate FROM Employee WHERE JoinDate > #1/1/90#;
See also: