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 the comments made by the listener when someone is taking a longer turn in informal conversation and telling an anecdote. These contributions by the listener (Really? Uh huh. That's amazing! Did you? Wow!) are short, need no reply or change in direction of the discourse, and are seen simply as expressing interest and encouraging the speaker to continue - not as an attempt to 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.