URL stands for Universal Resource Locator.
Most usually, when you hear URL, it means the web-address of a certain page. But in reality, HTTP URLs are only one of many types of Internet resources which can be referred to using URLs.
The following are all examples of HTTP URLs:
Like I said, there are many types of resources which you can refer to with URLs. Do distinguish these different types, URL schemes are used. Basically, they are just the keywords at the beginning of a URL line which mean a certain protocol used to access a resource referred by the URL.
So, http from the examples above is a URL scheme name. Other commonly used URL schemes you definitely know are: ftp, mailto and file. There are many more, but they're not used as often.
URL generic syntax
All URLs conform to the following generic syntax:
And this is the meaning each field of a URL has:
Indicate the protocol used to access a resource
Usually an IP address or a hostname of the server which has the resource
Location on the server which has the resource. Usually it's a hierarchical structure, like directory names for instance.
Most often, these are the dynamic parameters used to query a certain resource provider – it could a set of query parameters for the database or a script on a webserver you're trying to access.
A certain portion of the resource you're interested in. Quite often, it's the anchor of the HTML page you're trying to access.