In the UK, they're called "tax discs". They're little paper circles with the expiry date mm/yyyy printed on them. They sit in a circular holder that you have to stick to the inside of your windscreen (passenger side). Each year is a different colour, to make it easier for the police to spot expired ones.

It's an offence to drive or park your car on a public highway without a valid road fund license.

The Road Fund License (as the government likes to call it -- we call it Road Tax) costs between £55 (for a low-emissions, "alternative" fuel car) and £165 (for a high-emissions diesel car). That's yearly.

IIRC, I pay £165/yr for my car.

You can buy either a 6 month disc or a 12 month disc. If you take your car off the road, you can return the tax disc for a pro-rata refund.