Acronym for Integrated Services Digital Network. It is a set of standards defining a digital, synchronous subscriber access for voice and data to a telephone network. It defines the use of 64kbps bearer channels to transport data (such as voice). Actual interfaces can (and do) provide more than one bearer channel (usually 2 (BRI) or 24/30 (PRI)) refered to as B-channel which can be aggregated for improved bandwidth. For call setup/tear down, a call signaling protocol is used (Q.931 and its derivates, e.g. DSS1 or QSIG) is used between the terminal and the central office (sometimes also referred to as local exchange) on a dedicated extra channel known as D-channel. A basic rate interface (BRI) hence is often referred to as 2B+D.
From a VoIP point of view, ISDN is mostly seen as a trunk line interface connecting the VoIP system to the existing world wide telephony network. ISDN's data transmission capabilities are rarely, if ever, used.