RJ45 refers to a modular connector in the “registered jack” family; a format developed by Bell Labs, and adopted as a standard with oversight by the FCC. There are two primary varieties; “RJ45” refers to the IEC 60603-7 8P8C eight-conductor connector used for ethernet connections. (Though this common generic usage for “RJ45” really has nothing to do with the RJ45 standard and is therefore technically incorrect…)
The second variety, “RJ45S,” was a “keyed” connector (meaning that it could only be inserted into a specifically mating jack due to an extra tab on the plug) used with modem connections.