Based on size:

• Leaderboard banner ad (728 x 90)
• Full horizontal banner (468 x 60)
• Half banner (234 x 60)
• Square button (125 x 125)
• Vertical banner (120 x 240)
• Button (120 x 90)
• Button (120 x 60)
• Micro Bar (88 x 31)
• Medium Rectangle (300 x 250)
• Square Pop-up (250 x 250)
• Vertical Rectangle (240 x 400)
• Large Rectangle (336 x 280)
• Rectangle (180 x 150)
• Wide Skyscraper (160 x 600)
• Skyscraper (120 x 600)
• Half Page Ad (300 x 600)

Based on how ads are displayed:

• Traditional banner ad - most common, usually rectangular in shape
• In-line ad - displayed in a column at the right or left side of the web page
• Pop-up ad - found in a small window that appears over the web page you are visiting, can include Flash animation and/or video