Maps API
A maps API is a hosted service that serves the pieces a map needs over HTTP. Instead of running your own server, you request tiles and styles from an endpoint and authenticate with a key.
What a maps API serves
In practice you point your map at a single URL, the style:
https://api.maptiler.com/maps/openstreetmap/style.json?key=YOUR_MAPTILER_API_KEYThat style JSON names everything else the map needs, so the client fetches the rest on its own: the tiles, either vector or raster tiles, the font glyphs that labels are drawn from, and the sprite holding the style’s icons. Serving all four from one endpoint, behind one key, is what a maps API is for.
Hosted or self-hosted
Running the pipeline yourself gives you full control and no per-request fees, but you take on the imports, the updates, the storage, and the cost of servers and bandwidth. A hosted maps API removes that work in exchange for a subscription and a dependency.
Because the schema is open, this is not a one-way decision. A style written against the OpenMapTiles schema works against a hosted endpoint or your own server, so you can start on one and move to the other.