Look at the construction of a CMOS output. There are 2 transistors. One gets turned on to pull the output high (or 1) the other gets turned on to pull the output low (or 0).
An output like that can both sink and source current. Connecting more than one output together will cause problems if they are in opposite logic levels.
Open drain outputs (or open collector on bipolar logic) have a single transistor that pulls the output low. A resistor must be used to pull the output high. This sounds like an odd thing, however connecting several outputs together is possible, and the output will be low if any output is low. In addition, an open drain output can often be used to switch loads operating from a higher voltage than the logic.