A circuit board containing sockets into which other circuit boards can be plugged in. In the context of PCs, the term backplane refers to the large circuit board that contains sockets for expansion cards.
Backplanes are often described as being either active or passive. Active backplanes contain, in addition to the sockets, logical circuitry that performs computing functions. In contrast, passive backplanes contain almost no computing circuitry and simply act as a conduit to pass signals from one point to another.
Traditionally, most PCs have used active backplanes. In fact, the terms motherboard and backplane have been synonymous to some extent. Recently, though, there has been a move toward passive backplanes, with the active components such as the CPU inserted on an additional card. Passive backplanes make it easier to repair faulty components and to upgrade to new components.