Posters can be an alternative form of presentation. Rather than going through publication or a panel presentation at a symposium or conference, poster "sessions" usually occur at professional conferences in a separate location.
Posters are BRIEF -- no more than 300 to 800 words. Those words should be accompanied with graphs, tabular data, and photos if available. Thus, posters should be visually impactful.
Some poster sessions are juried. That is, they are weighed by a jury or panel of judges and must be formally accepted for presentation.
The authors of posters stand next to their posters at conferences and poster sessions, available to explain or defend their work to viewers.