Picture one flat surface in a spot everybody walks past, with paper pinned all over it. Nothing there is addressed to one person: whatever goes up is for whoever looks. Anyone can add to it, and it stays up until someone takes it down. Move that same surface into a computer and almost nothing changes — you still go to it, read what other people left, and put your own message up for the group.
可以想成一塊平平的板子，掛在大家都會經過的地方，上面釘滿了紙。板子上的東西不是寫給某一個人的，是誰看到就給誰看。任何人都可以往上面加東西，貼上去就一直在，直到有人拿下來。把這塊板子搬進電腦裡，幾乎什麼都沒變：你還是去看別人留了什麼，再把自己的訊息貼上去給大家看。
From the word’s history
The name is simply bulletin plus board: a short public notice, and the flat thing you fasten it to. Both meanings still do exactly that.
這個名字就是 bulletin（公告）加上 board（板子）：一則給大家看的短通知，和一塊把它固定住的平板。兩個意思到今天都還是在做這件事。
Packet Wiktionary entry kaikki:3f6f69a190036ae4:1098657620, etymology_text: From bulletin + board.
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