Every orchestra is different, but here are some instruments you're likely to see. 
string family
violin
viola [vee-OH-lah]
cello (violoncello) [CHEL-low]
bass (double bass, contra bass)
[rhymes with "face']
woodwind family
flute, piccolo
oboe, English horn
clarinet, bass clarinet
bassoon, contrabassoon
saxophones
brass family
trumpet
horn (French horn)
trombone
tuba
percussion family
timpani (kettledrums) [TIM-pa-nee]
vibraphone (vibes)
snare drum bass drum cymbals
tambourine triangle xylophone
marimba glockenspiel chimes
plus other things to hit, scrape, and shake
keyboards and harp
celesta [cheh-LESS-tah]
piano organ synthesizer
harpsichord harp
