#2 4chan was founded in 2003 by a 16 year old named moot. It was based on Futaba Channel (2chan.org), japan's largest image-based message board, and by based on, i mean moot downloaded the board software and ran it through google translate. 4chan, like 2chan, is a place where people are invited to post images (more so than text) on specific topics, divided into subboards which are headed by a single letter, like /a/ - Anime & Manga. 4chan based its boards on 2chan's, but in the beginning, it had way, way less. Back then it was mostly about /a/, which was ostensibly the point of the whole site, and /b/ - Random, which was affectionately referred to as 'the retard bin'. It's called /b/ because it was named after 2chan's catch-all board, also a /b/.
He first advertised the site on the forums of Something Awful, a place whose slogan is "the internet makes you stupid". This is actually really important to the history of 4chan. SA was and kind of still is the original bastion for sarcastic assholes on the internet. So the initial userbase is made up of caustic older nerds who have sort of a twisted sense of humor. They came for the anime, but they stayed because of 4chan's other unique feature: everyone posts anonymously by default.
To a 4channer anonymity is sacred and unquestioned. Their position is that by removing names you remove ass-kissing and circle-jerking and are left with the value of your content alone. Nobody cares about who you are and there's value in having discussion this way. However this had a few unintended side effects.
Anonymity means you can say whatever you want without consequence, so 4chan breeds aggressive, uncompromising behavior. Esstentially, people are much more forceful about their opinions, are more likely to call out others without provocation, and many use it as a platform to say things they'd never admit to believing in real life.
All these factors came together and had a profound effect on the larger internet. In the beginning, everyone was in on the joke, so /b/ was all about pretending to be stupid for a laugh, and in the other boards, passionate discussion went on in smaller numbers. But /b/'s "legendary threads" kept getting screencapped and shared about the internet, the site grew and added boards users demanded, ranging from the accessible, like video games, to the more niche, like hentai/alternative (for when people who are into H are telling you YOUR shit is too weird). Eventually, concepts that were culturally 4chan started seeping out into the wider internet, like Caturday, and the Rick Roll. Fun fact: the rickroll meme is actually based on the duckroll, an old 4chan troll that would seem to quote someone in a thread, but would actually take you to a completely different thread where someone had posted an annoying picture of a duck on wheels.
As 4chan exploded in popularity though, less and less people were 'in on the joke', so to speak. The signal-to-noise ratio plummeted and people started posting the most offensive, insane content they could, because that's what they thought being in /b/ was all about. Eventually this insanity started spilling into other boards, like /v/, which killed video game discussion there for a long time but gave us rage comics - which is one reason why reddit is so popular today!
Now, as 4chan is increasingly an internet-household name, moot's taken steps to beef up the moderating and cut down on the crap. The result is a 4chan that is a little more focused and usable, vs the complete mess it was circa 2010. Though /b/ will never be the original content machine it once was, 4chan is still a great place for fearless opinion-making. I visit both 4chan and reddit regularly because each offer unique perspectives on news. Reddit is always understanding and trying to see all sides, and 4chan doesn't tolerate any bullshit.
Yeah, you're gonna see some weird shit there. But it's a place built for weirdos. It might not be for everyone, but it's a big part of the internet.
EDIT: I just want to add a couple things.
If you're gonna go to 4chan, please lurk moar. The reason why it's gotten diluted is because people like to jump in, and 4chan culture is complex and historic. Plus if you say something stupid I and others will tell you to fuck off.
Second, don't bother going to /b/. So many people post there now (and by now I mean for the last like 8 years) that it's a total blur, no meaningful discussion can happen, and if it were to be destroyed, nothing of value would be lost. If you want to see memes being created as they happen, go to /sp/, and if you do do that, please let me know when you plan on visiting. And lastly, if the cognitive dissonance starts to give you a headache, remember this.
EDIT THE THIRD: Seriously! Don't go to /b/! /b/ is not 4chan! Similarly for those bitching about reddit, GET THE FUCK OUT OF THE DEFAULTS, nothing of value happens in most of those. People badmouth both these sites without investigating them to find the good stuff!