URIs and URLs
In practice you can use Type:URL for both URLs and URIs, but they are different. A URL string gives an address that guides how to access or locate the resource, while a URI address is an abstract notation for some concept.
For example, a wiki uses the URI "http://ontoworld.org/wiki/WWW2006" to denote the WWW conference 2006. Intuitively speaking, the URI thus does not stand for itself, but for this specific conference. URL-strings, in contrast, are mere lists of characters that have no abstract meaning. Still the URL of a page can be used as a URI that refers to that page (as an abstract document) as well.
Another important difference between strings and URIs is that the objects that are represented by some URI can be described further by using this URI in other statements. For example, we use the above URI of WWW2006 to specify information about the conference. It is not possible to specify information about a string in a similar way. Strings are just data and have no intrinsic properties other than being used in various places as concrete values for some properties.
The later observation also leads to problems: even the ontology language OWL DL we use as an export format uses many URIs for identifying its language constructs. But it is not desirable to make new statements about those language constructs. E.g. you clearly do not want to declare owl:sameAs, the property that specifies that two individuals refer to the same concept, as the capital of Germany! For this reason, the datatype Type:URL disallows certain URIs that already have a predefined meaning that cannot be used without changing the output format of the whole wiki. The corresponding URI ranges are specified by the MediaWiki message MediaWiki:smw_uri_blacklist which can be extended by wiki-administrators. URIs from those ranges will be rejected by the wiki, but this will not have any effect on normal usage where URIs typically just point to some web-resource, such as an HTML or image document.
The old datatype Type:URI from SMW versions prior to 1.0 is an alias for Type:URL in SMW 1.0.