There are two basic types of refrigerated container (reefer) which have each developed differently over the course of time:
Porthole refrigerated containers, also called insulated or Conair containers, do not have their own refrigeration unit. They are thus reliant on an external supply of cold air. Refrigeration units of various types, permanently installed on the ship, permanently installed in the terminal or clip-on units for individual containers, are used for this. (see below)
Integral refrigerated containers, on the other hand, have an integrated refrigeration unit, which is generally powered nowadays using a 3-phase electric power supply.
Nearly all statistics on containers refer to the "TEU" unit. TEU stands for "Twenty foot Equivalent Unit" and is used to designate a 20' container. A 40' container thus comprises 2 TEUs. Sometimes the unit "FEU" is used for 40' containers. 1 FEU is therefore equivalent to 2 TEUs.