URL

A path to external file, either local or remote.

Absolute path

file://
The file is located on local filesystem usually on hard drive, dvd or usb drive. Path to filesystem starts with / and no internet connection is required to load the file.

Examples:

file:///c:/documents/image.png
file:///c:/documents/page.html

http://
The file is located on http server and accessible by http protocol. Internet connection is required to load the file.

Examples:

http://www.server.com/documents/image.png
http://www.server.com/documents/page.html
http://www.server.com/documents/pagewithphpcode.php

Relative path

Part of path added to current location. / (slash) resets path to root, .. (2 dots) go one folder up, . (one dot) indicates current folder.

Examples:

Current location: http://www.server.com/documents/page.html
Relative path: image.png or ./image.png
Result: http://www.server.com/documents/image.png

Current location: http://www.server.com/documents/page.html
Relative path: ../image.png
Result: http://www.server.com/image.png

Current location: http://www.server.com/documents/page.html
Relative path: folder/image.png or ./folder/image.png
Result: http://www.server.com/documents/folder/image.png

Current location: http://www.server.com/documents/page.html
Relative path: ../folder/image.png
Result: http://www.server.com/folder/image.png

Current location: http://www.server.com/documents/page.html
Relative path: /folder/image.png
Result: http://www.server.com/folder/image.png