The Polkadot API Server is a wrap-around of the polkadot-js/api. This makes it easier to use the polkadot-js/api with any programming language in order to query data from Polkadot nodes. In addition to this, a number of custom defined calls were also implemented in the API Server. For example, one can query directly the amount of tokens a validator has been slashed at any block height.
The API Server was specifically built as a way for PANIC to be able to retrieve data from the Polkadot nodes that it will monitor. As a result, not all functions from the polkadot-js/api were included in the API Server. If you would like any endpoint specified in the RPC docs or the query docs to be implemented, kindly open an issue and we will consider adding it in a future release. You might also want to have a look at our contribution guidelines, especially if you want to try adding it yourself.
If you want to dive into the design and feature set of the API Server click here.
If you are ready to try out the API Server on your Polkadot nodes, setup and run the API Server using this guide.