Claircognizance is the least well known clair-ability.  It is clear knowing, and the users of such ability may find that they have fully formed ideas coming to them with little explanation, or no train of though that preceded it.

It is the receiving of knowledge and information subtlety through spirits or guidance.  As such, it is also the most overlooked and unknown ability to most psychics - as there is no spirit appearing to them or any fireworks to indicate it is working.

I haven't as easily dot-pointed this clair-ability as it is the most obscure, and what some people may describe as their "claircognizant" ability showing, it is for other people a combination of it and another ability, for example;

"People with claircognizance feel when a person is lying to them" - that could be attributed to clairsentience, etc.

It is quite simply, the acquiring of information from a higher plane.

To exercise this ability practice automatic-writing, and meditating to open yourself to receiving information.

Clairsentience is the ability to feel energy, or more simply it is the other sensory abilities.  It can be broken down into:

Feeling emotions - this is picking up on a spirit's or person's emotions, and feeling them possibly as your own.

Feeling energy - being able to pick up the energy of a spirit or a person around you, different from emotions as it is neutral, but present.

Psychometry - touching a person and reading them is clairsentience, as it requires the energy or emotion to trigger it.

Smells - smelling a scent of a spirit or deceased person was originally considered clairsentience, but it later grew into empathy and energy reading.