A Cougar is a much older woman who has a appreciation for the qualities of a younger sexy man. For example the fact they are like the energizer bunny and keep going and going and going, Oh My!
Another reason is there is no kissing to make it all personal, there is no I love you's to set up for a future disappointment and last but least the younger man always leaves so that the ladies can go on with their daily business. Nothing is ever said, it's always discreet and that makes the sex even more great!
A plus side also is that younger men are so much more sexier these days!
"see that MILF, she's a Cougar on the prowl!"
Meaning Her mom is looking for some fun with a younger guy.