D3DX_SURFACEFORMAT and Texture compression

I noticed that these are available flags to the D3DX_SURFACEFORMAT obj. but I could find absolutly nothing beyond this D3DX_SF_DXT1 S3 texture compression technique 1. D3DX_SF_DXT3 S3 texture compression technique 3. D3DX_SF_DXT5 S3 texture compression technique 5. does anyone know a resource when I can find additional information on this obj, or more important can anyone explain to me what these compression techniques are? Cheers! Kenny