Open-collector lines allow multiple devices to talk over the same line. Each output can be in two states: floating/hi-impedance (in which the pull-up resistors make the line go high), or low (in which the pull-up resistors are overridden). Any device on the line can set their output to either of these states and excessive current draw won't occur.
Interfaces that use open-collector communication:
Open-collector and open-drain are mostly the same.