A reefer container is a steel shipping container built to hold temperature sensitive cargo, most commonly a 40 foot high cube box with a refrigeration unit built into one end wall in place of a normal container door. It moves by ship for the ocean leg and by truck or rail chassis for the legs on either side, making it an intermodal box in the fullest sense: the same steel unit that leaves a packing shed on a truck chassis is craned onto a vessel, sails, and comes off the other side onto another chassis without the cargo inside it ever being touched.
It carries frozen meat and seafood, chilled, 2-8°C fruit and vegetables, dairy, some pharmaceutical bulk shipments and a range of temperature sensitive chemicals, and it is by a wide margin the cheapest way to move a large volume of any of that cargo across an ocean. Shipping lines own or lease the boxes, terminal operators handle them in port, and food exporters and importers build entire supply chains around a predictable weekly sailing schedule.
Plug points on vessel and at the terminal
A reefer container only works if it stays connected to power for almost the entire transit. On board a container ship it plugs into a dedicated reefer socket in a bay wired for exactly that purpose, one of several hundred such sockets a large vessel carries, monitored by a reefer technician who patrols the stacks checking temperature readouts and alarms through the crossing. Ashore, terminals run long rows of reefer plug points through the container yard, and a box comes off the crane straight into one of them, so the gap between vessel power and yard power is measured in minutes, not hours.
Generator sets for the road leg
Once a reefer container leaves the terminal on a truck chassis, there is no yard socket to plug into, so a diesel generator set, a genset, clips onto the front of the container and keeps the compressor running for the length of the road or rail leg. The genset carries its own fuel tank, and on a long drayage run from an inland factory to the port, or the other way from port to an inland distribution center, the fuel has to last the whole trip, because there is nowhere along most of that route to refuel a genset the way a truck stop refuels a tractor. Drayage trucking companies budget genset fuel and rental into the cost of every inland leg, since an empty tank stops the compressor as surely as a power failure at the terminal.
Controlled and modified atmosphere
Temperature is only half of what a reefer container controls on some cargo. Fresh produce like bananas, apples and avocados keeps respiring after harvest, using up oxygen and giving off carbon dioxide, and that respiration is what ripens and eventually spoils it. A controlled atmosphere reefer actively manages the oxygen and carbon dioxide levels inside the box, holding respiration down to a fraction of its normal rate, while a modified atmosphere setup simply seals the box and lets the produce's own respiration shift the gas mix over the crossing. Either way, the point is the same: slow the fruit down enough that a multi week ocean transit lands it at the other end still short of ripe, not already spoiled.
Weeks at sea, not days
An ocean crossing runs from a little over a week to several weeks depending on the route, far longer than any road or air leg the cargo will see. That duration is why a reefer container gets a full pre-trip inspection, a PTI, before it is loaded: the empty box runs through a full cool down and hold cycle, its door seals, drain plugs and data logger all checked by the shipping line's own reefer technicians or a container leasing company's inspection team, because a fault caught before loading costs nothing and a fault discovered mid ocean cannot be fixed until the ship reaches its next port.
Cheapest per unit, least forgiving of a fault
A reefer container is the lowest cost way, per unit of temperature controlled capacity, to move a large volume of cargo anywhere, and that is exactly why it carries the bulk of the world's frozen and chilled food trade. The trade off is recovery time. A refrigerated truck with a failing unit can divert to a depot within hours; a reefer container mid ocean is stuck with whatever fault it developed until the vessel docks, days or weeks later. That is why high value, short shelf life or highly sensitive cargo, most pharma and most fresh cut goods, still moves by air or a monitored active packaging system instead, and reefer containers stay the default for cargo whose shelf life can absorb a slower, cheaper, less forgiving mode.
Smaller volumes that need the same temperature band without the size of a full container still move in an insulated shipper instead.