SEER   

Seasonal Energy Efficiency Ratio (SEER), is most commonly used to measure the efficiency of a central air conditioner. It measures how efficiently a cooling system will operate over an entire season. The higher the SEER, the more efficient the air conditioner.

Technically speaking, SEER is a measure of equipments overall total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watt-hours) consumed during the same period

Energy Efficiency Ratio (EER) is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (95oF).

In technical terms, EER is the steady-state rate of heat energy removal (i.e. cooling capacity) by the product measured in Btuh divided by the steady-state rate of energy input to the product measured in watts. This ratio is expressed in Btuh/watt. The higher the EER, the more efficient the air conditioner.

AFUE :                                                        

Annual Fuel Utilization Efficiency: 

Used to Rate  Gas or Oil Fired Furnaces.

BTUH :

The British thermal unit (BTU or Btu) is approximately the amount of energy needed to heat or cool 1 pound of water 1 °F

British Thermal Units per Hour

The Furnaces ability to deliver heat is measured in BTUH

Example: 70,000 British Thermal Units per Hour

Equals 70K BTUH.

This would heat a moderate size house. Furnace Size is usually refferred to in BTUH

TONS AC:

A Ton of AC is 12,000 BTUH.

Example: 2 Tons is 24,000 BTUH, 3 Tons 36,000 BTUH