IP is Internet Protocol which is one of the major protocols that run the Internet. A protocol is a set of rules that say how people or devices behave and communicate. In the case of IP it is used to carry data across the Internet from one device to another and includes a method of how devices are addressed similar to how the street name and number of your house are determined.
We have been using version 4 of IP since about 1980 and it is referenced usually as IPv4. It had addresses that looked like 192.168.7.13 In the late 90s as the Internet and World Wide Web rapidly expanded it was discovered that there were not enough addresses in IPv4 for all the computers. At that time several methods were developed to stretch the addresses and NAT was one popular way of only having one address for the network that was recognized on the Internet and using addresses that were only recognized in the network but not on internet in the private network). Development of Internet Protocol version 6 (usually called IPv6) began at that time and slowly began implementing soon after 2000.
In the last year or so the major organization that gives out IPv4 addresses ran out as we ar again exploding the size of the Internet. Devices used on the Internet are in all places of the world and people are no longer limited to one or two Internet devices (and lots more coming). We now may well individually have one or more desktop PCs, a laptop or tablet and one or more cell phones and now our cars, houses, appliances etc. are connecting to the Internet.
With IPv6 there are 340 undecillion addresses which means about 50 billion billion billion addresses per person in the world. That should last us a while. The addresses now will use hexadecimal (base 16 number system which numbers can be zero thru nine and A thru F) and addresses will look like 2001:0DB8:8734:0001:7564:1234:7899:A3F6. In addition when a person or business is issued an address to set up a network they can have 18 quintillion devices per subnet and they can hav 65,536 subnets.
The number of addresses now available in IPv6 make our national debt look like pocket change.
For the foreseeable future the Internet will use both IPv4 and IPv6. IPv6 was designed to work with Ipv4. Gradually organizations will move over to IPv6 and if you were to look on your computer at home (use the IPCONFIG command at command prompt) you may discover that you are now using both IPv4 and IPv6. All network devices built for several years now have been designed for IPv4 and IPv6.
When I was at a Cisco conference in San Jose California a week ago one of the biggest items was getting us instructors comfortable with IPv6.