Building a character rig? One of the most fundamental concepts other than IK and FK itself, is a switch between the two. The FK/IK switch is an important feature for animators to have, so it is something that should be in the most basic of rigs.
If you are just getting started with rigging, you will immediately find that there are many ways to interpret the creation and function of an FK/IK Switch. A good place to start is likely the most classic of FK/IK Switch setups.
Have a look at Damon Lavenski’s multi-part tutorial for creating space-switching in Autodesk Maya, which offers a concise look into the basics of the technique. Lavenski currently is a CG Generalist for Psyop, working with modeling, texturing, rigging, animation and nCloth simulation.