I hear the term K1, K2, C1, and C2. What does it mean?

K1, K2, K4, C1, C2, C4, C6, OC1, OC2, OC6...

The letter indicates what kind of boat it is and the number is how many paddlers the boat is made for.
K = Kayak
C = Canoe
OC = Outrigger Canoe

C1 = Solo Canoe
C2 = Tandem Canoe
C4 = Four Paddler Canoe
C6 = Six Paddler Canoe

K1 = Solo Kayak
K2 = Tandem Kayak
K4 = Four Paddler Kayak

OC1 = Solo Outrigger Canoe
OC2 = Tandem Outrigger Canoe
OC6 = Six Paddler Outrigger

See how this works?