Difference between Poster Session and Oral Session
In an oral session, speakers present their work one-by-one in a series of short lectures (usually no more than 30 minutes, and potentially as little as 10 minutes). Each speaker presents, takes questions for a few minutes, and is followed by the next one. These presentations are usually in a room with a seated audience, in a lecture-style format. 

In a poster session, a number of presenters each prepare a poster (in a standard size) and mount them on boards in a large room. For a fixed period of time during the conference, all participants are invited to wander round the posters, reading and asking questions as they deem fit. Poster presenters typically stand by the posters and answer questions as people come by. 

For more information, see:


Talks vs. poster presentations: Which is better for advertising your research and building research networks?
How important are poster sessions in conferences?
What are some general good principles for creating a poster for a poster session?