Go to the official documentation here, on the right hand side is a list of the official lovelace card, click on it to get a picture how it looks like and a description how it works and all the “properties” you can set. For all the custom cards, you’ll mostly find a comprehensive guide on their respective Github page, which explains their card in detail.
There are already a lot of awesome frontends here on the forum, which can be used by everyone check out HKI, Dwains Theme, A Different Take on Lovelace.