from the FAQ:
There are many reasons you might want to use the Handle System, but one simple one is that you have information and other resources represented in digital form, sometimes called digital content, that you want users to access via the Internet and you plan to keep that content available over long periods of time. If the content's location is likely to have to change during that time, then you need a resolution system like the Handle System. You would assign your digital content unique identifiers, not just identify the objects by their locations. A location -- a given URL, for example -- is not a persistent identifier if the content moves to another location. Think about keeping track of a person. People are listed in a telephone directory by name. If you look up a person's name, you will find his address. If he moves across town, his address will change but not his name, so you will get his new address when you look up his name. If he didn't have a name, was known only by his address, and he moved, you'd probably lose track of him. If you tried his old address, he wouldn't be found. He'd have to tell everyone his new address, and hope they kept it. If he had a lot of friends, that might work, but it would take a lot of effort. Similarly, if your digital content is only known by its location, and that location changes, it will be hard for users to find it. If you give each object a unique name (an identifier), and associate that name with the object's location using the Handle System, you'd only have to update a single record with the new location, rather than notify everyone who might want to find the object, even if you could arrange to do so. The ability to change locations without changing names also applies to ownership. You can move or sell an object from one owner to another and still use the same identifier, which is very difficult to do using domain names that are required in URLs. Other reasons to use the Handle System are more technical, e.g., secure resolution and/or multiple resolution, for which we recommend you read the technical documentation found on this site.
The DSpace uses canonical handle system to guarantee unique URIs. By default it uses handle.net system: http://www.handle.net/introduction.html and we haven't subscribed to their server yet.
As an example let's consider following document URL: http://dspace.uabgrid.uab.edu/handle/123456789/19
On the same page there is canonical handle URI: http://hdl.handle.net/123456789/19 This URI will guarantee that above digital resource will be location/server independent. We haven't registered with handle.net to do this mapping, but we can do that. This will change '123456789' string in the handle.net URL.