An imageboard is a type of a web forum focused on posting images instead of textual communication. They originated in Japan with popular imageboards such as futaba and 2channel, the latter of which draws in over 3 million posts every day and makes 100 million yen a year for its creator Hiroyuki Nishimura. The most popular English-language imageboard is 4chan, though it doesn't do nearly as well with advertisers.
Imageboards classically use futaba's software or a modified version of it called Futallaby. This software is what gives imageboards their well-known URL hierarchy -- each subsection of the board is shortened to as few letters as possible and stuck after a slash to create simple web addresses like www.example.com/m/ (which could maybe be a forum to discuss movies). The letters are ingrained in the culture that imageboards have, to the point that most people would refer to that example forum as "/m/" instead of its real label on the site.
Most imageboards encourage anonymous posting, and the communities reflect this. Imageboards feel ephemeral because even though many of them have existed for over a decade, the culturally-enforced anonymity makes it feel like you're talking to no one. There's usually no way to keep track of who's who, so unless you're really into the community via something like IRC, you're unlikely to get attached to it as easily as with other types of forums where users have obvious personalities in their profiles.
Which leads to the obvious: imageboards are rude, pornographic, and absolutely full of trolls. See Also: YouTube comments. Anonymity breeds honesty, and that can be a good thing -- there are surprisingly a lot of decent people on imageboards, if you sift through the people pretending to be bigots -- but at some point you just get sick of it. For many people this rough culture must be what they like, since they keep coming back, but it gets very tiring very quickly and the culture is definitely the main thing stopping imageboards from ever being mainstream popular. They occupy a very particular niche.
Some imageboards, or sections of imageboards, may encourage posters to use tripcodes, which make the communication slightly less anonymous. A tripcode is essentially a password you input when posting, but they don't give the poster a name; they only give an identifier that's hard to remember, but can be used to determine for sure that two posts came from the same anonymous person. Usually tripcodes only get used when one of the posters involved is particularly relevant -- for example, the author of a story or a camgirl.
When imageboards shine is when they focus on a very limited subject and stay unpopular -- that's when they develop a tight community, start losing the anonymity a bit, and become bastions for subcultures like drug users, LGBT people, hackers, furries, people who like BDSM... I think these sorts of imageboards serve an important purpose, since they provide people with a safe space to talk about something society might otherwise ostracize them for.