The range of a voice or instrument is the set of pitches, from lowest to highest, that it can sing or play. A range can be described using the appropriate octave identification, for example, "from one-line c to two-line g". But it is often easiest to write the range on a staff, as the two notes at the high and low ends of the range.
A piece of music, or one performer's part in that piece, may also be described as having a range, from the lowest to highest note written for the performer in that piece. There is usually a difference (sometimes a large one) between the total range of the part and a smaller range that the part stays in most of the time (heading to the extreme highs and lows only occasionally). This smaller range is called the tessitura of the part. One can also speak of the tessitura of a performer's voice, which is the range in which it sounds the best (so that matching the tessitura of the part and of the performer is a very good idea). Notice the similarity between this second definition and the term power range (pg 89), sometimes used to describe the most powerful or useful part of an instrument's range.
A register is a distinctive part of a vocal or instrumental range. For example, singers may speak of the head register, in the upper part of their range, and the chest register in the lower part of their range. These two registers sound and feel very different, and the singer may have even have two distinct tessituras, one in each register. The large range of the clarinet39 is also divided into distinctive registers with different capabilities and very different timbres. Even when an instrument does not have a very large variation in timbre over its range, its players may speak of the difficulty of "playing in the high register" or a "dull timbre in the low register".