Posters are a common way to communicate science at conferences and other venues. They are often printed on a single large sheet of paper (e.g., A0), but size requirements vary between conferences.
You should use your poster as a visual aid to presenting your work - it should have the key figures, results, etc. that you need to show your audience to convince them of the main findings of your work.
At the same time, however, your poster should be able to stand alone: a person should be able to look at it, without you being there to explain it, and still understand the work that you have done.
However, a poster is not a manuscript, and you cannot simply copy/paste figures and text from your thesis onto a poster template. A poster is primarily about visual communication, so text should be kept at a minimum and figures should be kept simple and clear.
Figure 1.1: A poster is not a manuscript, from PhD Comics.
Every poster is different, but all posters should contain the following elements in some form. Boiled down to the essentials, the poster should inform your audience of why your research is important, the aims/hypothesis of your work, what experiments were done and how, and the overall conclusions/“take-home messages”.
There are many possible poster designs (just one example shown below), but it is common to use subheadings to divide the information on a poster, and to direct and focus the reader’s attention.
Figure 2.1: Example poster template, from Template Lab.
The titles of the subsections on a poster may not match the sections listed above. For example, it is best to have clear, specific, focused titles that state the take-home message of a section (e.g., “Drug X decreases lymphocyte survival” instead of “Results”).