These are also called ‘the strings’ in an orchestra. Look at the body shape of each wooden instrument, it is specially constructed to form a hollow sound box. The larger the instrument box, the deeper the sound. The music is produced when the 4 strings that are stretched over the sound box vibrate, resonating through the bridge to the wood and hollow cavity. The violin family instruments are played with a bow drawn across the strings or by finger plucking.

The pitch can be changed by modifying the tension or length of the strings. When a musician places a finger on the string, it acts as a temporary anchor preventing the vibration of the string to pass the fingertip, therefore it shortens the vibration length, plays a higher pitch than the string would play untouched.
Experiment: Press all of your fingertips together and place a small rubber band around them near the tips. Open your fingers to stretch the band in all directions. Pluck the band between fingers and listen to the differences of the longer looser lengths compared to the tighter shorter ones.
violin Violin
Made from several types of wood, the instrument is still very similar in design to those played in the 1500s. In fact, Italian instrument makers so beautifully refined the design centuries ago, they are still considered the best available; quite a few 400 year old Italian violins are still favored and in use today. Prized for its beauty, brilliance, range of expression and clarity of sound, the violin was the most important solo instrument of the 1600s. Many of the concerto compositions from the period are still very popular today. You will always see more violins in the orchestra than any other instrument.
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viola Viola
Larger than a violin, the viola has a flatter back and it produces a lower tone. Still smaller than its brother the cello, which produces an even deeper "voice", the viola is held under the chin like the violin. Also popular in the classical era, composers produced more sophisticated and complex arrangements to feature its warm, rich sound.
cello Cello
The cello, originally called a violoncello, rests on the floor by a retractable stem that helps support the instrument between the knees of the musician. It is like the violin and viola in shape, but is much larger, so it plays much lower octaves. Prominent as a solo instrument, the cello can produce marvelous and dramatic music and even approach the ranges of the violin by fingering very close to the bridge.
bass Bass
Also called the double bass, it is so large that most players have to stand or lean against a tall stool to play it. The shape slopes more at the shoulders of the sound box than the other stringed instruments, and produces a very low, deep resonance. Although it does not get as much spotlight solo attention as the other strings, it is considered the foundation of most orchestral music as the lowest tones anchor much of the melodies.