While anyone can run a node and a corresponding API, the Helium hosted version offers up the API through two endpoints.
https://api.helium.wtf — Beta, scalable endpoint for new features and tests. Currently connected to mainnet. This endpoint is used for feature development. Submitted transactions may get dropped.
https://api.helium.io — Stable, scalable production service. Connected to mainnet.
The API routes are grouped together in sub-pages of this page. Refer to the subpages for details.