An orchestra is considered to include four main instrument families: woodwinds, brass, strings, and percussion. This is how the orchestral instruments are arranged on a stage, with minor exceptions, and how the performers' parts are arranged in the conductor’s score, except for soloists, whose parts may be given special treatment. The keyboards may be considered a separate section of the orchestra, as may the plucked string instruments, depending on their role in the piece. Sometimes, a keyboard or plucked string instrument will be grouped with percussion. The following is a basic description, recognizing that the particular instruments and numbers will vary depending on the date and style of music being performed.

The string section consists of the bowed string instruments. This is the part of the orchestra that most usually distinguishes it from other ensembles, particularly a band. The strings have four types of instruments, but five sections: first violins; second violins; violas; violoncellos, familiarly called cellos; and double basses or contrabasses, also known as string basses. The number of players in each section depends on the overall size of the orchestra, but the numbers are balanced between sections.

The woodwind section includes reed instruments – both single and double reeds - and those which make sound from air passing over the mouthpiece. This section of the orchestra often has only one or two players on each instrument, again depending on the orchestra size. There may be, for example, two flutes, with one performer doubling on piccolo; two oboes, with one performer doubling on English horn or Cor Anglais, if necessary; an Eb clarinet, two Bb clarinets, and a bass clarinet; and two bassoons and one contrabassoon. Depending on the piece, other instruments may be included, such as saxophone.

The brass section, like woodwinds, have their sound produced by air, but the brass sound is created by the vibration of the performer’s lips as she or he blows into the resonating mouthpiece. The brass section generally includes four French horns; two or three trumpets; two or three trombones, including both tenor and bass trombones; and tuba. Sometimes, one also finds cornets or Wagner tubas, and possibly euphoniums.

The percussion section includes instruments that make sound when struck with hand or mallet or when struck together, like cymbals. The percussion section of the orchestra often includes timpani; chimes or tubular bells; celesta, harpsichord, or piano; concert harp; other pitched percussion, such as glockenspiel or xylophone; snare drum; and auxiliary percussion, such as triangle, tambourine, bass drum, and cymbals.

The keyboard section of the orchestra includes piano, celesta, harpsichord, organ, and harmonium. Which of these – if any – is employed will depend on the date and style of music.

The plucked string section of the orchestra includes the concert harp, guitar, mandolin, banjo, zither, and other plucked instruments that may have a role in a particular piece. The harp is the most often used.