What is Reddit?
Reddit is an entertainment, social networking and news website where registered community users can submit content, such as direct links, text posts, images, and so forth, making it as an online bulletin board. These users are called Redditors, can vote submissions up or down to organize the posts and determined their position on the site.
In 2005, Reddit was founded by two University of Viginia roommates, Steve Huffman and Alexis Ohanian.
The site's mascot is an alien, called Snoo.
Reddit Culture in detail
Getting positive karma (upvote>downvote ratio is higher) and a upvote/downvote is a huge deal. If karma will be too low, you can be restricted from posting for some time.
All users can browse other user post history. If you say anything contrary to reddit consensus some people could get downvote all your posts and it could be brought somewhere unrelated.
Mods can get you shadowbanned.
Majority of website content was taken from somewhere else.
How Reddit culture differs from imageboard culture
There isn't any karma/votes system on imageboards.
Only Board Owner and Board Volunteers(Mods) can look at user posts history. Your opinions stay with you only for one thread.
There aren't any shadowbans on imageboards. Unless you count user filters as shadowban.
Original content is created, when users want to make them.