Eurocard is a European standard format for PCB cards, which can be plugged together into a standardized chassis. The chassis consists of a series of slots on the top and bottom, into which the cards are slid so they stand on end, like books on a shelf. At the "back" of each card is one or more DIN 41612 (or other DIN) connectors, which plug into similar connectors on a backplane at the rear of the chassis.
Eurocards have standardized sizes based on the 'U', for unit. 1 U is 44.45mm, and cards come in various multiples of this size. A 3U Eurocard is 100mm x 160mm with one DIN connector on the short edge. A double or 6U Eurocard is 216mm x 160mm with one or two DIN connectors on the long edge. A triple or 9U Eurocard is also defined, but not often used.
Eurocard does not define the signals or the meanings of the pins in the DIN connectors, this is left up to the individual implementation to provide. Several systems have used the Eurocard standard mechanicals as the basis for a complete bus system, including VMEbus, VXIbus and CompactPCI. There are also innumerable "one-off" applications.
The measurements have also been used to define the 19-inch rack standard, although in a roundabout way. In this case the U's measure the vertical height of the equipment, with the width and depth being that of a 6U card. For instance, a 3U card is 150mm high, by 215x160.