You play Phil, a young fish swimming in Interweb Bay. Phish Guru (or Father, depending which edition you play) teaches you how to find food and duck dangers around InterWeb Bay. Use the mouse to move around the bay hunting for worms to eat. Before you can eat the worm, you must decide whether to eat it or reject it based on its URL. Press "E" to eat or "R" to reject.
You earn points for eating healthy worms and avoiding the bad ones. If you're not sure about a URL, press the "T" key so Phish Guru will teach you. The site uses URLs of well-known sites such as eBay, Amazon, National Geographic, banks, and so on. Of course, a URL isn't always a simple as www.nationalgeographic.com -- it could be www3.nationalgeographic.com (it's edible).
After finishing round one, Phish Guru teaches a lesson. Lessons cover URLs having several parts and whether Wombankonline.com and wombank.com are legitimate. Plenty of valid URLs go beyond the www.url.com address or use a variation like the Wombank example. These lessons show how you can figure out which one is legitimate -- if not both.