Let's start with RJ45 first.

RJ stands for “registered jack,” and the true RJ45 connector was developed as part of the standardized telecommunication network interface for the purpose of connecting telephone networks. It is smaller and cheaper version of telephone communication network and it is easier to work with them as well.

8P8C: refers to the array of pins and the name is Eight Position, Eight Contact. In 8P8C connectors, each plug has eight positions that are spaced approximately 1 mm apart. Individual wires are inserted into these positions. There are a variety of 8P8C connectors  with the modern RJ45 Ethernet connector being the most prevalent. So 8P8C is a name of category and RJ45 is one of the subcategories. 

The differences between 8P8C connectors and RJ45 connectors are shown in the following diagram:

The connectors are not compatible with one another because of the keying used in RJ45; 8P8C connectors can plug into RJ45 receptacles, but the inverse is not possible as it used Poke Yoka to avoid wrong connection.

Today, almost all electronic device using 8P8C connectors that show documentation for RJ45 connectors, which is technically incorrect. This is a common mistake in throughout industries, and actually it is not a big concern as long as both terminology works. 

For better understanding of RJ45 let's talk about two types of RJ45 wiring.

There are two RJ45 pinouts (T568A and T568B), and  they are technically 8P8C connectors, it is very likely you will hear them strictly referred to as RJ45 connectors, and all documentation surrounding them will use the same terminology. As such, for the sake of clarity, we will refer to them as modern RJ45 connectors.

As you may know T568A and T568B is about wire arrangement on RJ45 plug connector and keystone jacks.