Anyone can create their own customized Minecraft maps and share. A custom map usually involves a mission the player has to go through. For example it could be an adventure map where you start off stranded on an island and have to make your way through to survive and finally get to the mainland by building a boat by collecting materials you find scattered around on the island. That’s just one example of couse and a quite basic one. At the same time adventure maps can be a lot of fun far from everyone enjoys them and therefore there’s a large variety of different categories you can choose from.
Parkour is a very popular type of map. Here you have to make your way through a map by climbing and jumping from block to block. It’s a lot of fun as it’s not that easy and involves a lot of risk-taking. Parkour maps can be played alone but still be fun while adventure maps and similar can get boring playing all alone as it’s usually recommended playing with buddies when making your way through an adventure in Minecraft.
There are multiple sites giving you the opportunity to download and play their maps. Minecraftblogs.com is one such a site offering you a wide variation of Minecraft maps. All of these maps are of course for free and usually contributed by its memberbase or just random people wanting more people to find their maps and play them.
Don’t forget the texture packs. Some custom made maps usually comes with separate Minecraft texture packs you can download at the same page you are downloading the map from. This is because certain textures doesn’t fit with certain maps and it can really bring a lot of more joy if using the right pack.
Hopefully this has given you a better insight of what a Minecraft map and how you can make the most out of it to enjoy it!