A single-handle disk faucet is made with a pair of ceramic or plastic disks encased in a cylinder. The upper disk rotates when you turn the handle. Water flows when the inlets of the upper and lower disks line up.
Getting the parts
If water wells up around the handle or drips out of the spout, replace the seals with exact duplicates. If that does not solve the problem, replace the entire cylinder -- you cannot open it to replace disks. If water flow is slow or erratic, particles of rust and minerals may be clogging the inlets. Cleaning them solves the problem.
Less than an hour for most repairs
Screwdriver, hex wrench, cleaning brush
Shutting off water, dismantling a faucet, installing small parts
Shut off the water, close the sink stopper, and place a rag in the sink to catch any parts
Repair kit with seals and O-rings, silicone grease