Display Advertising is just advertising but in the Online Space, so needs to be planned just like off-line.
There are loads of different sizes, shapes, types and a growing list of creative options available. Web 2.0 allows page changes without refreshing, allowing much more flexibility which is a pretty handy feature too.
Taking a visit to www.flashtalking.com/showcasepage.php will give an outline and links to various types of ads you could utilise, click through each for examples:
Here are some 'Leaderboard' size ads that LSBU have placed online in the past, specs are 728 (w) x 90 (h):
This one is called a 'Super Leaderboard', specs are 970 (w) x 90 (h):
..and here's a standard MPU, specs are 300 (w) x 250 (h):