The string section consists of instruments of the violin family: violin, viola, cello and double bass. The first violinist of an orchestra is its concertmaster. A harp or two is also included in the orchestra if required by the music being performed.
The brass instruments of an orchestra are, from lowest to highest, tuba, trombone, French horn and trumpet. Brass instruments are a type of wind instrument, with either valves or slides (the trombone).
There are several woodwind sections in an orchestra. The flute section has a flute and a piccolo (highest tones in the orchestra), the clarinet section includes the clarinet, bass clarinet and E-flat clarinet (highest in the section), the oboe section includes the oboe and English horn, and the bassoon section includes the bassoon and contrabassoon.
A timpani is a fixture of an orchestra's percussion section. You'll also see various other percussion instruments such as cymbals, bells, triangle, snare drum, bass drum and xylophone.
Besides instruments regularly included in an orchestra, soloists might play instruments such as piano or classical guitar while being accompanied by an orchestra.
A symphony orchestra can have over 100 players, with the string section making up about two-thirds of the instruments. Woodwinds are next, with about 16 players, followed by brass, with about 14 or 15 players. There's usually two or more percussion players, depending on the piece being performed.