I understand that some of the differences between boards can be a little bit daunting. To help with this, I'm going to go over some of the more common boards and their alternates, what makes them different. I'll try my best to be fair to both sides, but I do have my biases, I will admit.
If my biases shine through a little too much to you, let me know in the meta thread.
/v/: Aims to be a celebration of 4chan's /v/ board culture, with some frowned upon aspects of it from 4chan's /v/ being allowed. Community wise, there isn't much discernable difference between 4chan's /v/ and 8chan's /v/, aside from 8chan's /v/ allowing generals and discussion of industry politics.
/svidya/: At first, it was created as an answer to a perceived rapid quality drop in /v/. Post quality would be enforced, and other guidelines such as forbidding SJW discussion were put in place. However, the board died, was claimed by someone else, and made into a more lax video game board. Unlike /v/, board culture is not allowed; it's just for discussing video games. It's far slower than /v/ as well.
/games/: As the board's subtitle says, /games/ is basically an union between /v/ and /tg/, the board for discussing traditional games. As a direct consequence, every kind of videogame is allowed to be discussed, including visual novels, tabletops, quests, and all form of traditional gaming. The only exceptions, and this comes from the intent of raising post quality, are games witch fanbases are known for being troublesome, like Minecraft, Overwatch and Undertale. So the main direction of /games/ is to keep high-quality gaming discussions, and this is shown also by their other general rules: shitposting and metaposting are confined in a meta thread, blogposting or trips are fine only when strictly needed, and low quality kind of threads tipical of 4chan's /v/ are not allowed.
/a/: Wants a community similar to 4chan's /a/, however, on 8chan's version, the unwritten rules on 4chan's /a/ are actually enforced by moderation. Post quality is enforcable, and low quality posts can get you banned.
/animu/: Views /a/'s approach as far too heavy-handed and limiting. /animu/ has more lax rules regarding posting quality and also allows "funposting" or lighthearted shitposting as it could be called. /a/ has a more elitist view on the type of discussion allowed on their board which can be observed from their hatred for "crossposters" (people who both post on /a/ and other boards) which is often used as a boogeyman in discussions had there and seeks to have a distinctive culture from all other boards, /animu/ on the other hand welcomes the general board culture seen on most of 8chan. Certain type of threads that are highly frowned upon on /a/ like recommendation threads or picture dumps are allowed and threads/posts are less prone to being removed for being of supposed low quality. instead only enforces 8chan's global rules.Tripcoding is NOT allowed.
/b/: Only moderated by global mods, and the only hard rules are the global rules and that MLP and pedophile discussion threads are stuck within a general. However, the mods handle things like /b/ did under W.T. Snacks. Meme bans can happen, since no rules for the users means no rules for the mods, too.
/1/: Made in response to a banwave that hit /b/. Made to be completely hands-off in regards to moderation, and has only one thread to post in. It moves way slower than /b/, but has a very different vibe.
/pol/: The vast internet right wing conspiracy. The board's largely of a far-right and fringe-right mindset, and brought with them the sense of paranoia that 4chan had; prepare to be called Jewish or a shill if you disagree with the general zeitgeist. They embrace the /pol/ identity, unlike 4chan, where it's just a joke.
Compared to 4chan's /pol/, it's much more activist-minded, takes itself more seriously, and is less humorous. However, it seems to have a very good working relationship with 4chan's /pol/, and there's a lot of crossposting between the two boards.
/midpol/: Seeks to be a more general-purpose politics board for the site, allowing for a broader range of discussion. Took off in response to dissatisfaction with /pol/'s moderation and community. The user base has more respect for dissent, debate, and alternate viewpoints, though the board is understandably slower than /pol/. Unlike /pol/, they don't seek to moderate for post quality, or chase out those with political views they disagree with.
I will admit I'm a bit ignorant about these two, not being Christian and all.
/christian/: The original Christian board on 8chan. Tends to have a lot of interdenominational banter/infighting, and also has a reputation for raiding/"breadpilling" other boards. More of an imageboard-esque atmosphere. Doesn't allow for debate and the moderation appears to favor Catholics.
/christ/: Slower, but openly welcomes other denominations and debate. Has less interest in infighting, but has more lax moderation than /christian/. The atmosphere is less imageboard-like and more discussion-oriented.
/n/: Created by Hotwheels due to his frustration with a string of bad moderation decisions by /pol/. News stories posted here appear on 8chan's front page. It's run by global moderators, therefore moderation is somewhat minimal.
/news/: Focuses more on news in various spots around the world. Very slow compared to /n/, but still has a group of people that frequent it.
/realnews/: Run by the creator of /killcen/ and /leaked/, this board acts more as a personal news aggregator and newspaper published by the board owner, with his opinions on the news at hand. Unlike the other boards, he runs it more heavy-handedly, forbidding "psyops" and race baiting, expecting intelligent discussion from his readers. Every page and video shared is archived or mirrored in some form, and he has bi-monthly downloads of the information he accumulates linked.
These two aren't alternate boards as much as partner boards for one another, even claiming to be part of the same community.
/monster/: Only allows monster girls to be posted. Think of it as comparable to /h/ with monster girls exclusively interacting with human males.
/chaos/: Allows monster futanari and boys to be posted, as well as girls. Comparable to /d/, with /h/ as well, allowing monster boy/monster girl pairings and monster boy solo stuff.