A public board. Anyone can read it — people in your class, people who are not, and anyone who wanders in off a search result. Assume a teacher is reading over your shoulder, because one day one will be.
There is a word filter and it is not subtle. It will also catch you spelling things creatively.
Messages are stored on the server with the time and the IP address they came from, so that abuse can be dealt with. Anything posted here can be removed. See privacy.