A kayak is a craft that is paddled with a twin-bladed paddle, while the paddler is in a seated position. The paddler typically has a spraydeck around the waist to keep water out of the vessel, but that is not necessary the case.
In the case of K1, K2 and K4 racing kayaks, the K stands for “Kayak” and the 1, 2 and 4 stands for the number of paddlers that the kayak was designed for.
Recreational kayaks general don’t have a number to show how many paddlers it can take, although single and tandem kayaks are both very popular.
A canoe is paddled with a single-bladed paddle, which has a T-grip on the other end of the shaft, and the paddler typically kneels down in the canoe.
In the case of C1 and C2 racing canoes, the C stands for “Canoe” and the 1 and 2 stands for the number of paddlers the canoe was designed for.
Open canoes generally don’t have a number to show how many paddlers it can take, as it depends largely on what the canoe is being used for. The same open canoe could be used to carry a family of 4, or to do a solo multiday expedition.
In South Africa, it is common for paddlers to talk about racing kayaks as “canoes”. In fact, most racing kayak clubs are called “canoe clubs”, which doesn’t help to put the record straight.