A Uniform Resource Identifier (URI) is a specially-formatted name, such as an Internet address, that identifies some name or resource. The format of URIs is described in RFC 3986, which updates and replaces the earlier RFCs 2732, 2396, 1808, and 1738. A Uniform Resource Locator (URL) is an older but still-common term, which basically refers to a URI corresponding to a resource that can be accessed (usually over a network) in a specific way.
Here are some examples of URIs (taken from RFC 3986):
ftp://ftp.is.co.za/rfc/rfc1808.txt https://www.ietf.org/rfc/rfc2396.txt ldap://[2001:db8::7]/c=GB?objectClass?one mailto:John.Doe@example.com news:comp.infosystems.www.servers.unix tel:+1-816-555-1212 telnet://192.0.2.16:80/ urn:oasis:names:specification:docbook:dtd:xml:4.1.2
This manual describes the url library, an Emacs Lisp library
for parsing URIs and retrieving the resources to which they refer.
(The library is so-named for historical reasons; nowadays, the “URI”
terminology is regarded as the more general one, and “URL” is
technically obsolete despite its widespread vernacular usage.)