Contributed by B@rt.
Armatures are part of the new character animation tools that are available since Blender 2.20. They replace the old IKA functionality - IKAs from old Blender files can still be loaded into 2.20 but you cannot edit them anymore.
To learn how to set up a model with armatures read the following tutorial by WP:
https://members.chello.nl/~wvanoverbruggen/animtools.html
To lean how to animate it using the new non-linear animation system, read (also by WP):
https://members.chello.nl/~wvanoverbruggen/actions.html
The new system also contains a new IK (inverse kinematics) solver; to learn how to work with this read:
https://elysiun.theliberation.com/tutorials/animation/
More basic information is available here:
https://members.yourlink.net/gruff/bones/index.html