Soil texture classifies soil based on the proportion of clay, silt, and sand particles. Sand is composed of large particles, silt is medium particles, and clay is small particles. These particles can be separated by suspending a soil sample in water and then allowing the particles to settle. The soil texture is determined by calculating the percentage of each particle type in the sample and then using the USDA soil texture triangle for identification. See this post to learn how to use the soil texture triangle.