What exactly is a URL and what is it good for? The short answer is that it is the address you use to find information on the internet, and it stands for Uniform Resource Locator. The technical answer follows, but it is pretty easy.
The internet is made up of thousands of computers known as Servers, all connected by phone lines and network cables. Servers are simply programs running on computers which have information that they are sharing. Different kinds of servers offer information in different formats, and to view each type of information your computer needs two things: a client, and an address.
A client is simply a program which understands the type of information the server is providing. You want to view web pages, so you use a Web Client...Safari, Internet Explorer, Netscape Navigator etc.
The address is the URL. The url has three pieces to it: 1) The type of server you want to talk to, 2) The name of the machine, and 3) the name of the document that you want from the machine.