If you ping each of those, you'll see that www.yahoo.com hits a different server than www1, www2, etc. Not sure why they have different domain names set up like that, but that's all they are.
They do this for load balancing purposes. This way they won't have 10,000 People hitting the same server at the same time. Each user gets a server assigned on a round robin type of queue.
(it is actually more complicated than that, load balancing sw/hw usually tries to determine which server is less busy and and of course up and running