"The tenor is a type of male singing voice and is the highest male voice within the modal register. The typical tenor voice lies between the C one octave below middle C (C3) to the A above (G4) in choral music, and up to high C (C5) in solo work. The low extreme for tenors is roughly Bâ™­2 (two Bâ™­s below middle C). At the highest extreme, some tenors can sing up to two Fs above middle C (F5). Within opera, the lowest note in the standard tenor repertoire is A2 (Mime, Herod), but few roles fall below C3 (one octave below middle C). The high extreme: many tenor roles in the standard repertoire call for a "tenor C" (C5, one octave above middle C). While some operatic roles for tenor require a darker timbre and fewer high notes, it is generally accepted[who?] that any tenor should be able to sing with a full timbre up to an G4. In the leggiero repertoire the highest note is an F5 (Arturo in I puritani), therefore, very few tenors can have this role in their repertoire. Within musical theatre, most tenor roles are written between Bâ™­3 and Aâ™­4, especially the romantic leads, although some fall as low as Aâ™­2 (such as Anthony in Sweeney Todd) and others as high as D5 (such as Man #1 in Songs for a New World." "Within Choral and pop music, singers are classified into voice parts based almost solely on range with little consideration for other qualities in the voice. Within classical solo singing, however, a person is classified as a tenor through the identification of several vocal traits, including vocal range (the lowest and highest notes that the singer can reach), vocal timbre, vocal weight, vocal tessitura, vocal resonance, and vocal transition points (lifts or "passaggio") within the singer's voice. These different traits are used to identify different sub-types within the tenor voice sometimes referred to as fächer (sg. fach, from German Fach or Stimmfach, "vocal category"). Within opera, particular roles are written with specific kinds of tenor voices in mind, causing certain roles to be associated with certain kinds of voices." Source and further information: http://en.wikipedia.org/wiki/Tenor

As an appendum to iwnit's excellent answer, I would say tenor range is (standardly) C5 (top C) to A2. High tenors like Florez are capable of much higher, (probably to E5, even F5) though in leggiero repertoire, F5 is most commonly performed in a mezza-voce production: i.e. a falsetto-y sound. (Pavarotti as a lyric (lower) tenor sings the high F thus.) You can tell if you're a tenor because the high C will always be (if singing well) 'full' voice. If you're a baritone, most likely the high C will turn into mezza-voce. Singing well, this will happen absolutely naturally with no jolting 'switch' in registration; it is the cords easy transition into the new voice because they can go no further in full voice. That is not to say that high baris cannot sing high C in full voice, but most likely it will be a little strained. Moreover, a baritone's vocal timbre will be much 'darker' or 'warmer' than the tenor voice and going up to high C (or even high B (B4)) will suddenly change in vocal timbre to be (as one of my teachers put it 'tenorial'). In a true tenor voice, however, there will be no fundamental timbre change.