For SharePoint Online, API support is a bit of a mess. On one hand, developers can directly access most REST-based APIs from https://{tenant}.sharepoint.com/_api/, and when they develop solutions, this is often a valid way of directly communicating with SharePoint.
All APIs available through REST are documented at https://dev.office.com, and a more comprehensive walk-through of constructing the API addresses is found at https://dev.office.com/sharepoint/docs/sp-add-ins/get-to-know-the-sharepoint-rest-service.
Figuring out the API locations is quite easy, as they all follow the same patterns. The base address (https://{tenant}.sharepoint.com) is always the same. By adding /_api/, we can then choose what scope of data we will need to access. The available scopes are the site for a site collection and the web for a single SharePoint site. They map directly to the server-side object model naming conventions, where SPSite is the site collection and SPWeb is a single site...