IP is the Internet Protocol, as defined by STD5. That's actually
IPv4, with IPv6 extending the address space from 32 to 128 bits and
doing a lot of other things. IP by itself is not a very interesting
protocol, since it does so little---only defines packet format with sender's and receiver's
address and a few other things. The only non-trivial feature of the
protocol is handling fragmentation,
which becomes less and less needed with wide PMTU deployment. Source routing is largely defunct in
today's Internet, so we won't even mention it as a feature.
IP is, however, one of the few things that all Internet hosts have
in common; it's the glue network protocol.
Traditionally, IP has had best-effort delivery semantics, and it
certainly still carries best-effort connotations, though lately it has
acquired QoS features.