The Instruments of the Orchestra
Instruments in an orchestral score are grouped by families and always appear in the same order, from top to bottom:  woodwinds, brass, percussion, and strings.  Sometimes there are one or two harps or a piano in the orchestra; if so, they will appear between the percussion and strings.  The order of instruments within each family is always the same and is shown below.  
With woodwind and brass instruments, there are usually 2, 3, or 4 of each instrument in the orchestra, and each instrument has its own part.  Woodwind players usually can also play smaller or larger versions of their instrument when needed.  These are listed in parentheses after each instrument.  Many instruments in the woodwind and brass families are transposing instruments; they sound at at different pitch than the one that is written.  
With string instruments, a group of instruments all play the same part.  The size of the group depends on the size of the orchestra.  In a large orchestra there can be as many as 18 first violins, 16 second violins, 12 violas, 10 cellos, and 8 double basses.  Smaller orchestras have fewer strings than this but there are always more first violins and fewer of the lower instruments.
The Order of Instruments in an Orchestral Score

Woodwind Instruments
Flute – sounds as written  (piccolo  – sounds one octave higher than written)
Oboe – sounds as written  (English horn – in F; sounds a fifth lower than written)
Clarinet – in B flat; sounds a major 2nd lower than written, or in A; sounds a minor third lower than written (bass clarinet – in B flat; sounds a major 9th lower than written)
Bassoon  - sounds as written (contrabassoon – sounds one octave lower than written)
Brass Instruments
French Horn – in F; sounds a fifth lower than written
Trumpet  - in B flat; sounds a major second lower than written, or in C; sounds as written
Trombone - sounds as written
Tuba - sounds as written
Percussion Instruments
Many different instruments can be played by the same player, including timpani (large drums that can be tuned), various other drums, cymbals, mallet instruments (xylophone, marimba, vibraphone) and other instruments that are struck to produce a sound.  Often there are two or more percussion players in the orchestra.  One may play just timpani.
String Instruments
Violin I
Violin II
Double Bass