In general, unmodified thin-set mortar is a blend of Portland cement, sand, and water retention agents that is mixed with water by the user. The appropriate product standard for unmodified thin-set mortars is ANSI A118.1. Look for this standard on the packaging (e.g., product meets ANSI A118.1 when mixed with water). Unmodified thin-set mortar may also be referred to as dry-set mortar by the manufacturer.
Modified thin-set mortars are similar to unmodified thin-set mortars, but include additional polymers such as latex. The additional polymers are introduced in either liquid or powder form. In the first case, the user mixes an unmodified or dry-set mortar powder with a liquid polymer additive instead of water. In the second case, the polymers are added by the manufacturer to the dry blend in powder form. The user then mixes the powder with water. The appropriate product standards for modified thin-set mortars include ANSI A118.4, ANSI A118.11, and ANSI A118.15.