A communication channel can be simplex, in which only one party can transmit, full-duplex, in which both correspondents can transmit and receive simultaneously, or half-duplex, in which the correspondents alternate between transmitting and receiving states (such as conversing adults). Even though the channel might be capable of supporting full-duplex communication, if the corresponding entities are not capable of transmitting and receiving simultaneously, the communications system will be half-duplex (as in the example of the conversing adults).
Communication between two entities can be considered either in-band or out-of-band, depending on context. In-band communication is communication which occurs via the primary channel between the communicating entities. Out-of-band communication occurs via an alternative channel, which is not considered to be the primary channel between the entities.
Which channel is primary and which is an alternate depends on context and the existence of an alternative channel. In the case of a conversation between two people, the primary channel could consist of verbal communication while the alternate channel consists of visual body language. Of course, if emotions rise, these two channels might reverse roles, with body language becoming the primary channel!