Yoong Liat wrote:I often see the word 'poster', which is not found in my dictionaries. I wonder why it is so commonly used.
There speaks a prescriptivist!
You should look further. The American Heritage Dictionary gives:
poster
NOUN:
1.
1. A large, usually printed placard, bill, or announcement, often illustrated, that is posted to advertise or publicize something.
2. An artistic work, often a reproduction of an original painting or photograph, printed on a large sheet of paper.
2. One that posts bills or notices.
One posts entries in ledgers and notices on noticeboards (online or not)