The Internet Protocol, which controls how machines talk to and address each other. Usually you have TCP or UDP layered over the top to provide port-based addressing and checksums...
The most common version of IP at the moment is version 4, though you may well also see version 6 about from time to time (mostly at very large ISPs though.)
[Need more info on the difference between IPv4 and IPv6...]
If you need to manipulate internet addresses, read Handling internet addresses. You may also be interested in the DNS page for turning hostnames into addresses.