A turn is the time during which each participant in spoken discourse holds the floor. it may be short, as in the two turns below :
A : What's the time?
B : 9 o'clock
or long - eg when someone is giving a speech.
The changeover between participants is known as turn taking. Turn taking is rule governed . In short turns, the speaker will indicate when s/he has finished speaking by such paralinguistic devices as intonation, drawl on the final syllable, or may actively bring other participants into the conversation :
A : ..... Didn't something similar happen to you once, Jean?
Speaking without receiving such signals consitutes "interrupting" and may be seen as impolite. The exception to this is back-channel language - eg when someone in taking a longer turn in informal conversation and telling an anecdote. These contributions by the listener are seen simply as expressing interest and encouraging the speaker to continue - not as an attempt to t"take over the discourse.
In formal, long turns - speeches, presentations, lectures etc - it is usually conventionally accepted that the speaker has the right to finish what s/he is saying without interruption unless s/he overtly signals that s(he has come to a point where participants may contribute :
A : OK, that's the marketing policy. Does anyone have any questions before I go on?
This will often also happen at the end, will a signal of closure and a "handing over" of the turn :
A : Ok, that's all I have to say. Thank you for being such a nice audience. Does anyone have any final questions or comments?
I've used the word "conventionally", and turn taking is often affected both cross-culturally and within communities by factors such as gender. See the further reading below for more information.