The orchestra is perhaps the most recognizable figure within the classical music genre; it's an instrumental ensemble that plays classical music and its many off-shoots. A counterpart of the orchestra, the symphony orchestra, is a variation of the ensemble that plays only symphonies -- although in more modern times, the term symphony orchestra refers to any large orchestra, even one that isn't exclusively symphonic. At any rate, the orchestra is an integral part of any local music community, one that provides endless entertainment (and even employment) for those most interested in the form.
The basic difference between an orchestra and a band is the instrumentation - a band almost never has stringed instruments such as violins, violas, and cellos. It often, however, has either an upright bass or an electric bass. And a band typically plays music that is "less classical" than an orchestra. Many bands play mostly marches, while other bands play pop and jazz oriented music.
An orchestra is comprised of four instrument groups: strings, which feature violins and cellos; brass, which features French horns, trumpets and trombones; woodwinds, which features flutes, clarinets and bassoons; and percussion, which features any number of drums (including snare, bass and timpani) in addition to the piano or sometimes harpsichord. The number of individual musicians within an orchestra depends largely on the orchestra in question, its location and its type. An orchestra in a large city will often require a large number of employed musicians (the orchestra is, after all, a full-time job) of which only a fraction will play at any given performance; an orchestra as such will sometimes have 80 or 90 musicians on the payroll with only 40 or so seats for performance. It's not uncommon for an orchestra to keep several musicians on hand that aren't necessarily regular members of the orchestra; sometimes a special instrument not found within the roster will be required for a piece, in which case the orchestra will call on one of their in-the-wings musicians. Additionally, some larger cities will feel the need to include more than one orchestra in the area, all with different names; it is here that a symphony orchestra and an orchestra will utilize the name difference without any real style switch.
