MMario: it is very difficult to describe, but my best offer is that only those with "Absolute Pitch" can identify a single note in isolation. With most people (as with Nicole's comments above) it is a matter of "Relative pitch", i.e. you have a basis for comparison.
One of the best examples of 'relative pitch' which most people will be able to identify is when listening to a favourite album. If you are singing along, you know the pitch at which the next song starts without thinking about it. You may not know the note name, or whether it is a Third, or a minor fifth, above the last note of the previous song, but your subconscious has the details. If you stop the album before the song starts,and start singing, you will usually find that when you re-start it you are in tune. However, try to sing the same song later without the advance clues and you will usually find yourself off key.
Hope this clarifies.

Nigel