Patreon is basically a blog that requires readers to be logged in and pay for access to each blog. Creators usually use Patreon in conjunction with another public content distribution platform, like YouTube.
They handle payments for creators. They keep the list of patrons and what tier they're in, and they have some integrations so you can automate some types of rewards for your patrons.
Their premium service adds some new stuff, like handling merchandise.
Patreon is just a website and database with a blog. I read once they have 300 engineers, and can't figure out why...
They're not, but apparently they're trying to so they have a justification for increasing their cut.
Patreon started as a way to tip / nanofund artists whose content you enjoy, because subbing a buck of 5 to a writer or drawer/painter is easy.