Core Materials
The core affects the sound and bow response of the string.

Steel Core - Some qualities of steel core strings include quick bow response, high elasticity, powerful projection, longevity and stability in tuning and tone.  Steel core strings tend to sound lively and bright.

Synthetic Core - Synthetic strings are designed to reproduce the warm sound of gut strings with the advantages of synthetic materials, which include stability and resistance to temperature and humidity.  Big and powerful tonal volume, brilliance, intensity and warm tones are some of the characteristics of synthetic core strings.

Gut Core - Gut strings produce the darkest, warmest tone.  They exert high pressure on the bow, resulting in big tonal volume.  Pizzicato players appreciate their natural, less harsh sound.  Because gut core is a natural material, it is sensitive to temperature and humidity and not as durable as steel or synthetic core strings.

Gauge

Manufacturers use different terminology in describing string gauges.  Soft, light, medium, heavy and thick are a few terms used to describe gauge. 

Soft, Light, Dolce, Weich, Thin - Lower tension, low bow pressure, and a brighter sound.

Heavy, Thick, Stark, Forte - Higher tension, more bow pressure, and a more powerful sound.

Medium, Mittel - The most popular string gauge used because it balances the tension and sound qualities. 

String Materials

Common materials used in string production include Titanium, carbon steel, chromesteel, silver, tungsten, and gold-plating.  Here are some advantages of various string materials.

Titanium - Lightweight and yields a warmer tone.

Carbon Steel - Extremely durable

Chromesteel - Easy response and a lively sound.

Tungsten - Used to increase volume output.

Gold-plating - Primarily used on violin "E" strings for its sweet solo characteristics.