Temptation is something you want to have or to do, even though you know you shouldn't. That bag of peanut butter cups on top of your fridge might be an example of a temptation.
The thing that you want despite knowing it's not good for you — like the cool sneakers you really can't afford — is a temptation. The feeling is also a temptation — so wanting those shoes is temptation. The Latin word temptare, or to taste, is where temptation comes from, which makes a lot of sense when you think of that bag of peanut butter cups calling to you from the kitchen.