What is GPS RTK?
Nate 
What is RTCM?
RTCM is an acronym for Radio Technical Commission for Maritime. This governmental body came up with a way to communicate positions for boats and other vessels many decades ago. Technically, RTCM is just a protocol. We, however, will be using the term RTCM to mean the bytes of correction data related to GPS timing anomalies.
The contents of RTCM frames can be decoded but you, the user, rarely need to. Instead you simply pass the bytes to the GPS receiver and it will parse the correction data.
There are a few different types of messages but the ones we care about are numbers 1005, 1077, 1087, and 1230. Each message type has a different length but as a rule of thumb it's a couple hundred bytes every second. Each RTCM message contains details about the GPS/GNSS network, and perturbations in the ionosphere and troposphere.
Remember, the GPS satellites are very far away; about 20,000km or 12,000 miles away. A lot can happen to the signal from the GPS satellites to you across that distance. Geomagnetic storms cause slight timing delays increasing the location error. Earth's gravitational field is not uniform so relativistic effects can add inaccuracies. If we know the second to second issues within our local vicinity, a RTCM capable receiver can correct the location solution with great precision.