Names for types of URLs
Type 1 is just a "URI" (sometimes called an "absolute URI").
For types 2, 3 and 4 the definitive answers are in RFC 3986, section 4.2.
They are all "relative references", but according to the RFC are qualified thus:
../images/icons.png- "relative path reference"/images/icons.png- "absolute path reference"//.../icons.png- "network path reference"
The latter is often used if you want to specify a URL containing a domain name, but where you want the protocol to match the protocol used to access the current resource. For example, if your images are downloaded from a CDN, you could use this to default to https if the current page was also downloaded via https, thus preventing the warning about including non-secure resources in a secure page.
number 3 is also considered relative. number 4 is absolute, but lacks the protocol. This is useful, if you want to be able to access the same URL using HTTP and HTTPS.
Absolute URLs specify the location of a Web page in full, and work identically no matter where in the world you are.
Relative URLs are context-sensitive, giving a path with respect to your current location.
- Absolute http://www.example.com/images/icons.png
- Document-Relative ../images/icons.png
- Root-Relative /images/icons.png
- Protocol-Relative //www.example.com/images/icons.png
For #4, I've also often called them "Protocol-Agnostic"
Type three is root-relative.
Dunno about 4.