The IP is the address of the server.
The webserver, in this case Apache, can divide itself up so that it hosts many domains/subdomains using the same IP.
When you type in yoursitename.com in the address bar, the browser finds the IP and goes to that machine, but includes the name of the site it wants in the headers. So, Apache knows which site to route the request to.
If you just enter the IP address, Apache is not given the name of a domain, so it serves up a "default domain".
Think of the IP address as the address of an apartment house complex. If you go there with a resident's name in hand, you can look up and find the apartment number. If you go there without the name, you are routed to the managers office.