In addition to the document below, the API documentation is hosted here.
When viewing the API documentation linked above it is important to note that there are two versions (V1 and V2) of the API and both are active. As older resources are depreciated, newer versions are available in V2. At the bottom left of the web page there is a selector for which version you are displaying documentation . (see image below)
Download the Web Services document below as a pdf here.