4chan is an anonymous imageboard website created by Christopher Poole in October of 2003. The website features a plethora of different boards, each pertaining to different interests. Some notable boards are /b/ - Random, /pol/ - Politically Incorrect, and /g/ - Technology. What makes it different from other forums like Reddit is that registration isn't required (or available). It is completely anonymous, and every user is identified as Anonymous. On most boards, you're able to specify a name and a tripcode. Tripcodes help others identify you, since anyone can use your name. A few boards like /pol/ and /biz/ (politics and business, respectfully) give each user in the thread a unique string of character to identify them.
Note the string above with the orange background. That's for identifying a user on some boards. Also notice the numbers on the top right beginning with >>. Those are replies to that post.
4chan has put together lots of schemes and plots to take down websites over the years. One of it's most infamous is "Project Chanology". This is where users from the website, commonly referred to as "Anons", went to the Church of Scientology and protested outside. Most of them would wear Guy Fawkes masks, which became the go-to symbol for Anonymous.