To my mind there are several kinds of naps, with a nap being any sleep outside of the normal go-to-bed-at-night-and-get-up-in-the-morning sleep.
Chair Nap Any nap where you are more or less upright in position. Chairs, couches, and airplane seats all provide locations for chair naps. These are usually more of a doze than actual sleep; often times you are dimly aware of what is going on around you and you cycle between wakefulness and sleep.
Couch Nap Couches support two kinds of napping, upright and supine. Laying down on the couch usually leads to a deeper nap, perhaps even longer. Sometimes an afghan or blanket is employed for warmth or comfort. Very satisfying overall.
On the Bed Nap Moving on from the couch we have naps taken on the bed. Here pillows can be employed, and you are already conditioned that this is a place for sleeping. These naps can be several hours and are quite decadent. Laying down in the middle of the day? Luxury.
In the Bed Nap At the top of the nap hierarchy is a form of napping barely distinguishable from overnight sleeping. All clothing is removed and whatever is worn to bed is put on. You nap under the sheet and covers. These are serious naps, usually taken when ill or extremely tired. However, on occasion they can be quite restorative and rejuvenating.
Today I had a short chair nap, followed by a longer on the bed nap with blanket. Very nice indeed.