What is sculling in swimming?

Sculling is a type of rowing where the rower controls two oars (vs. sweep rowing when each rower has one oar). A scull is the shell of a boat that is rigged (set up for) sculling.

A boat for one person, a single, is also commonly called a scull. The two Words are used synonymously.
a "scull" is a type of rowing boat. A sweep oar boat is where each rower has only one oar, on alternate sides down the boat, and a sculling boat is one where each rower has 2 oars, one in each hand. A sweep rowing boat can hold 1, 2, 4 or 8 rowers, and a sculling boat can hold 1, 2 or 4.