People have been playing instruments together since there were instruments to play, but as classical music became more complex and more instruments were invented during the Baroque and Classical periods, the layout of these groups if instruments also became more complex, resulting in the modern orchestra which we see today.
Two centuries ago a typical orchestra might have included around 30 musicians. Today, that number can be over a hundred. Each family of instruments is assigned its own position, as shown in this handy diagram.
Front and centre, with their back to the audience, so that the orchestra can see them, is the conductor. His or her job is to direct the orchestra as a whole to ensure that they make beautiful music rather than a godawful racket.
The strings make up the largest single group of instruments and are arranged at the front of the orchestra. There might be 30 violins, divided into two groups (the First and Second Violins), and positioned to the left of the conductor. Closest to the conductor will be the First Violin: s/he is the leader of the orchestra. In addition there will be around ten cellos and a similar number of violas, arranged to the right of the conductor. At back right will be the double basses, typically around eight. The strings make up such a large proportion of the orchestra because they are quieter than the other instruments: if you don’t have enough strings then you won’t be able to hear them properly.
In the centre, behind the violas, are the woodwinds, with flutes, oboes and sometimes a piccolo in front and clarinets and bassoons behind them. Often these musicians will be capable of playing several instruments, and may change over part-way through as the music demands.
Behind the woodwinds and next to the double basses is the brass section, with French horns closest to the woodwinds, then about three trumpets and trombones, a saxophone or saxophones if the piece requires these, and a tuba to the right. Again, these musicians are often skilled with several instruments.
Finally, behind the violins to the conductor’s left are the percussion instruments. Unpitched percussions such as the timpani (kettledrums), cymbals and drums are positioned by the brass section, and to the conductor’s far left are the pitched percussion instruments, which may include a harp or harps, the celesta, or the glockenspiel.
What about the piano? The red-headed step-child of the orchestra, the poor old piano is sometimes excluded entirely. When a piano is included its position will depend on its prominence in the piece/s being performed. If the music is for piano with orchestral backing the piano is likely to be a featured soloist and the piano will be in front of the violins to the left of the conductor. This is also the position assumed by any visiting soloist, presumably because they’re part of the reason the audience is there so people want to see and hear them. Otherwise, the piano is sent up the back with the other pitched percussion, possibly to think about what it’s done wrong.
And there you have it: one orchestra assembled for your listening pleasure.