The clarinet family much resembles the Von Trapp family in organization and range, as well as sheer size - there are five types of clarinet regularly in use in orchestras today.

Clarinet in Bb, Clarinet in A

Written as transposing instruments and notated in treble clef, these clarinets dominate the upper register of the clarinet's normal duties in the orchestra. The Bb clarinet is the most often used (in military bands, wind bands, and orchestras alike) and as such is beginning to do away with the demand for the A clarinet in some cases. The level of virtuosity inherant in most clarinet players today makes it possible for them to just carry around a Bb and then transpose the parts which either are still in A or slip into it when the composer calls for the player to double. The clarinet in A will always be around and available because of the great body of literature that has been composed specifcally for its more melodic tone quality, but it is less likely to see players constantly switching back and forth between the two during a performance today than it would have been in the 1960's.

Clarinet in C

Although Beethoven (as well Schubert, Bizet, Liszt and others) wrote for the C clarinet in his first and fifth symphonies, it has had to struggle to stay along side its transposing brethren. The C clarinet was used much in the way the Bb and A were, in regards to the tonality of a piece, so pretty much anything until 1860 in C or C minor would probably have a part written for C clarinet. The clarinet itself has a more individual, brusque quality than the other clarinets which makes it good for literalism and melodic development. Players will usually just transpose the part these days, if it isn't already transposed.

Clarinet in D, Clarinet in Eb

The Eb or "little" clarinet has a long history and an assured future. Its size and timbre are totally unique and yet in demand. The shrill biting quality it sports made it invaluable in opera pits and wind bands, and as such it made its way into the symphony orchestra. For being a secondary instrument there is an enormous amount of literature for the Eb.
The D however has not enjoyed the same success. Wagner and Mahler and Germans in general seemed to be fond of it, and Strauss even invokes it in his famous Till Eulenspiegel. Two possible reasons for its departure from the mainstream could be the lack of literature and the fact that it sounds remarkably similar to the Bb.

Of note, there are also much smaller clarinets in F and high Ab but these are usually found today in European military bands where the instruments have been passed on through generations of players. Also, Mozart wrote for a B natural clarinet in Cosi fan tutte in his original score, but this instrument, if it actually existed, has not been seen for a few hundred years. The standard versions of Mozart's score today replace the B natural clarinet with a part for the A clarinet.

The generally accepted written ranges for these instruments is E3 to G7 or G#7 with some adventurous (or ignorant) composers writing even the high A7. The disadvantage in writing for the clarinet in this extremely high register is that the tone becomes distorted and overly-penetrating while the intonation goes all to hell. It might be playable but it is not advisable.