Guitar strings are one of the most important components in a classical guitar's sound and feel. Strings can be made out of several materials and in various thicknesses. The thickness of your guitar strings not only effects the way that they feel under your fingers, but also the amount of tension applied to the neck of the guitar.
String Types
The most common materials used in the creation of guitar strings are steel and nylon. Steel strings are used on electric guitars (so the magnetic pickups can read the vibration of the strings) and full-size acoustic guitars. Classical guitars, however, should not be strung with steel strings. A full set of steel strings applies too much force on the neck of classical guitars, potentially damaging the instruments. Instead, nylon strings or mixed strings (nylon strings on the bottom end, steel plated copper wound strings on the high end) tend to be used on classical guitars.
String Tensions
With steel strings, string tension tends to be measured by using the actual gauge (in inches) of the string. Although some nylon string sets also list the gauges of the strings, they generally use the terms light, normal or high tension. Light tension strings are thinner strings that apply less tension to the neck than the other types. Normal tension strings apply what is generally considered a standard amount of tension on the guitar neck. High tension strings are thicker strings that apply more tension to the neck than the other types of strings.
Feel
Beginning guitarists may like the feel of normal tension strings better than the feel of high tension strings. The thickness of high tension strings digs into the fingers more than that of normal tension strings, potentially hurting a beginner's fingers. High tension strings tend to work well on an instrument with low action, so if your classical guitar is set up with low action, you may prefer the feel and responsiveness of high tension strings.
Sound
Thinner strings, like low or normal tension sets, tend to sound sweet with more rounded lows and mids than high tension sets. Vibrato tends to sound better with thinner strings as well. High tension strings, however, have a bright sound perfectly suited for flamenco music.