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The answer to this almost certainly depends to some extent on where in the world you are, but the general idea will be similar anywhere you go.

There are, broadly, two types of heads aboard a boat, and what type you get often depends where you're intending to go. The first is a permanent tank type - the head flushes either directly or via a macerator into a permanent septic holding tank on board - more often found on seagoing vessels. The other is a cassette type (effectively a smaller, interchangeable tank), more often found on river or canal boats.

The process for emptying your tanks, whether cassette or onboard, is only slightly different. If you're using cassettes, you'll take them ashore and empty them in dedicated facilities and usually rinse them out before returning them to the boat. With a permanent tank, you'll use a pumping station ashore, connect a hose to a fitting above deck, and pump out. Pumping stations all do the same job, but they can differ significantly - do what the owner of the facility tells you to, and read the instructions! It's also worth rinsing these out when you're done, simply by flushing some water through the system and pumping it out.

If you're in the US, BoatUS has [an article](https://www.boatus.com/magazine/2016/august/pump-outs.asp) you should read with more information in it.