Khan is the "control plane" and thread-runner vane. Its main purpose is to allow external applications to run threads via a Unix Socket and receive the result.
At this stage, Khan's external interface is still experimental, and there are not yet proper libraries for other languages that can make use of it. Therefore, these documents will only touch on Khan's internal interface.
Khan's internal interface lets you run threads via Khan rather than having to poke Spider and subscribe for the result. This interface is simpler and more ergonomic than Spider's, so is usually preferable.
There are two tasks for running threads from userspace: %fard and %lard. The former is for running a thread from file and the latter is for running an "in-line" thread, where you pass Khan the thread directly.