1) Bowed Strings
The four principle orchestral string instruments are (in descending order of overall pitch) the violins (usually divided into two sections, playing individual parts), the violas, the cellos and the double basses. Each have four strings arranged in order of pitch, can be played by means of a bow (arco) or plucked (pizzicato), but whereas the violin and viola are played with the instrument resting between the shoulder and the chin, the larger cello (or, to give it its full title, violoncello) is placed facing outwards between and slightly behind the knees, and the bulky double bass is played standing up or seated on a high stool. Enthusiasts of Medieval, Renaissance and early Baroque music will encounter earlier varieties of bowed instruments known variously as vielle, viol, or in its earliest form, fidel (hence the modern nickname for a violin, 'fiddle'). The most popular member of the viol family is the cello's precursor, the viola da gamba (literally 'viol of the legs').
2) Woodwind
The four principle woodwind instruments of the orchestra all work by means of a system of keys (usually silver-plated) which when variously depressed and released allow air to pass through differing lengths of the instrument resulting in notes of different pitch. In order of descending overall pitch, these are:
Flute
a normally silver-plated (or in more extravagant cases, gold), narrow-bored instrument, held horizontally just under the mouth, and activated by blowing air across an aperture at one end of the instrument. Its higher-pitched cousin, the piccolo, is often encountered, although the lower alto flute rather less so. Early forebears include the unkeyed fife. The most popular close relation is the recorder family, largely unkeyed and end-blown in the vertical position.
OboeOboe
a narrow-bored wooden instrument descended from the medieval shawm, held vertically, and activated by means of placing the end-positioned double-reed in the mouth, and blowing under high-pressure so as to force air between the two bound reeds, causing them to vibrate. Other members of the oboe family include the lower pitched cor anglais (or English Horn), and (far more rarely) baritone oboe and heckelphone (bass oboe). The instrument's most famous predecessor is the Baroque oboe d'amore, often used by Johann Sebastian Bach.
Clarinet
like the oboe usually wooden, played vertically and held in the mouth, but with a wider bore and consisting of a single reed which when activated vibrates against a detachable mouthpiece. The standard instrument can be pitched in B flat (usually) or A, and the family is unusually extensive including the higher-pitched E flat, the B flat bass, the rarely-used C, the alto (a modern relative of the basset horn), and the even more obscure double-bass or 'pedal' clarinet. Occasionally the clarinet's 'popular' cousin can be seen in the concert hall, the saxophone.
BassoonBassoon
as the name would suggest, the bass member of the woodwind family, and by far the largest, especially its lower-pitched relation, the extremely bulky double or contra-bassoon. Like the oboe, it is a double-reed instrument, although to facilitate the playing action (the instrument is normally held across and in front of the body) it is connected to the bassoon via a silver-plated, curved crook. Its most notorious cousin is the Baroque serpent, shaped very much as its name would suggest.
3) Brass Instruments
Brass instruments are also activated by blowing into them, although instead of using a form of reed over which the mouth is placed, the lips are placed against or inside the cup of a metal mouthpiece, and made to vibrate against its inner rim. In order of descending pitch, these are:
Trumpet
one of the most ancient of all instruments. Played horizontally via a series of valves on the top of the instrument which are opened and closed in various combinations to create different pitches. Occasionally, the piccolo (higher) or bass (lower) trumpets are heard (and the trumpet's 'popular' cousin, the cornet), although more common nowadays in 'authentic' Baroque orchestras (which use instruments of the correct period or copies thereof), is the 'natural' or valveless trumpet. The more notationally limited bugle is rarely heard away from its traditional military context.