All the cornets except one are standard Bb instruments. The exception is the soprano cornet, in the key of Eb. The Bb cornets are divided into three sections, the solo cornets (front row), and the second and third cornets (back row). Also, one standard Bb cornet is called the Repiano cornet. In a "standard" brass band, there are four solo cornets, two 2nd cornets, two 3rd cornets, and a Repiano (rep). Youth bands however tend to have more than these numbers, Youth Brass 2000 currently has seventeen Bb cornets.
(Trumpets aren't used in a brass band!) These are split into several "families", but they all use the same type of instrument, in the key of Bb. They are basically split into three sections: the front row; the back row consisting of the second and third cornets; and the somewhat "specialised" cornet, the "repiano" or rep. Cornets are sometimes used in orchestras or wind bands
Front Row (Solo Cornets)
These sit in a row behind the front row. They tend to play the harmony against the front row's melody, or are used to accompany other instruments in the band who have the tune at that moment. Some composers use them as a sort of "alternative front row", so no excuses that you aren't good enough for the front row if your band plays some of these pieces!
As with the second cornets, there are usually two or three of them, and again they are used for basic harmony. They often have a simpler version of the harmony that the second cornets are playing.