How do floor registers work?

Most homes are heated and cooled by one central air system. The system is composed of a furnace, an air conditioning unit, and ductwork. When it is summer, you turn off your furnace and begin to use your air conditioner. The cold air is transferred through your home by air ducts. For efficiency and uniform room temperatures, air is moved back to the air conditioner through cold air returns as well as floor registers and wall registers that you place at the entries to these air ducts. When winter comes, you turn off your air conditioner and turn on your furnace, and the cycle continues only this time with warm air through the vent covers.
Very often, we have exceptions for this central air system. We may not want a room to get warmed up as it is rarely used and thus save some money on the gas bill. We may also not want to cool down a basement because it is usually already cold and we may also save some money on the electricity bill. These are most common examples of why someone would want to control the air flow in their house. To allow or disallow the air to go through the vent covers, we simply shift the position of the lever on our floor registers and wall registers and circulation just keeps going without wasting unnecessary effort for keeping the uniform temperature in that room.