How to navigate the Nether

• Crimson forest. These biomes are red, red, red. Here you'll find mobs like piglins and hoglins, and tree-like plants that can be used for wood in crafting recipes. Watch out for piglins if you're not wearing gold, and hoglins if you don't have warped fungi.
• Warped forest. Warped fungi? They're a part of this very gloomy biome, full of blue-green hues and endermen. This place is safe(ish), making it one of the better places to live. There are more wood-but-not-quite-wood trees here, and warped fungi that are loved by mobs like striders but hated by hoglins.
• Basalt delta. Love choking on ash and burning in pools of lava? Well then basalt deltas are for you! These chaotic (and strangely beautiful) biomes are loved by mobs like magma cubes. You won't find builds or structures here, but you can find blackstone, a dark grey block that can substitute cobblestone in crafting recipes, and basalt.
• Soulsand valley. Do you want skeletons, ghasts, fields of soul sand and soul soil, and mesmerizing blue fire that deals double damage versus normal fire? Soulsand valleys look cool but these places are dangerous. Unless you get the soul speed enchantment from piglins.
• Nether wastes. Another awesome new biome? Not exactly. Nether wastes are the new name for all the old boring parts of the Nether you're familiar with. Find zombified piglins, nether fortresses, and even the new bastion remnants here.

Related: How to find all the new biomes in the Nether Update

Locations

• Bastion remnant. Big building, filled with piglins and loot, if you're okay with probably pissing off all of the piglins when you open their chests. It's huge, messy, confusing, sort of cool looking, and filled with a species that only likes things that are covered in gold.
• Ruined portal. It's a nether portal, but ruined very ruined. Find obsidian, gold, maybe even a chest here. You can also find... crying obsidian? It's like obsidian, but glowing and used to craft a respawn anchor.

Mobs

• Piglin. Villagers in the Nether? Oh, not they're just piglins. You can trade with them if you have gold on you. And if you're wearing gold. And if you don't destroy any gold or open any chests around them. It's a tenuous relationship. Piglins also hunt hoglins, so they love crimson forests.

Related: How to find piglins in the Nether Update

• Hoglin. Big aggressive monsters that can be bred with crimson fungi, butchered for pork, and scared with warped fungi. Baby hoglins like baby piglins, but adult hoglins and adult piglins are constantly fighting. Hoglins aren't friendly unless you're holding warped fungi or are standing near a nether portal.

Related: How to find hoglins in the Nether Update

• Zombified piglin. Remember zombie pigmen? They're now zombified piglins, and they have floppy ears. Other than that, they're pretty much the same. They hang out mostly in the nether wastes and are neutral unless you attack one. Than every zombified piglin is your enemy.
• Zoglin. Trying to entice hoglins to the Overworld for your farm? Fat chance, it's now a zoglin! These zombified hoglins are ultra-aggressive, and not happy at all to see you. Hoglins belong in the Nether.
• Strider. It's probably not a good idea to bring a horse into the Nether. What about a strider? These guys love lava pools, and by tying a piece of warped fungi to a stick and using a saddle, you can actually ride one around yes, even through lava.

Related: How to find striders in the Nether Update

Netherite

• Replaces diamond as the best ore in the game. Harder to obtain, more durable, deals more damage, gives more protection, can have higher-level enchantments. It's better than a diamond in every single way. Oh, it's also fire and lava proof.
• Found in the lowest levels of the Nether. Find ancient debris in the lowest parts of the Nether, and must be mined with a diamond or netherite pickaxe. No exceptions.
• Ancient debris becomes netherite scraps. Throw your ancient debris into a furnace or blast furnace to get netherite scraps. Now we're cooking!
• Combine netherite scraps with gold ingots for... netherite ingots! Using your handy dandy crafting table, combine four netherite scraps with four gold ingots to get exactly one netherite ingot. No one said this would be easy.
• Use a smithing table to create some seriously powerful gear. Netherite ingots can be combined with diamond gear to create netherite gear! This gear is better in every way, and things like custom names, enchantments, and existing durability levels are preserved when upgrading.
• Find netherite ingots in bastion remnants. If you're seriously lucky, you have a small chance of finding netherite ingots in bastion remnant chests. That is if you can get around the hordes of piglins.

Little changes in the Nether Update

The little changes. Smol bois. I'd think of more but I have to keep this section short.

Blocks and items

• Respawn anchor. You want to live in the Nether. You will die in the Nether. Can't respawn in the Nether though because beds explode in the Nether. That's alright, just grab some crying obsidian from ruined portals and throw it together with some glowstone. Boom, respawn anchor!

Related: How to use a respawn anchor in the Nether Update

• Lodestone. Lodestones allow you to redirect compasses from pointing North to pointing at, well, the lodestone. This is useful for finding your base after a long journey, by just following the compass until it points you back to the lodestone. You'll need netherite though.
• Target block. Want to practice your aim, or create a needlessly complicated contraption that accomplishes your goals at a distance? Target blocks are for you! These blocks register where projectiles hit them, and can power redstone contraptions as well.
• Soul fire items. There's no coal or charcoal in the Nether, so you have to improvise. Or, in my opinion, improve! Soul campfires and soul torches are crafted using soul soil, and burn with an intense blue flame. Pretty hot if you ask me.
• New crafting blocks. Need cobblestone while in the Nether? Too bad, it doesn't exist. You can use blackstone in all the cobblestone recipes, though. Need wood? Use crimson or warped stems instead. Don't try to burn them for fuel, however, as it won't work.

Music

• New music tracks composed by Lena Raine. Lena Raine, the composer for indie game Celeste, made new music for the Nether Update. They're beautiful, haunting, and various other adjectives used to describe good game music.
• The songs play dynamically in different areas. Different songs play in different biomes, so every area gets a new atmospheric tone that wasn't present before.
• One song can only be found on a disc in bastion remnants. One of the songs, a funky tune known as Pigstep, can only be found on a disc in bastion remnants.

Other small things

• New visual effects and transitions. I'll sum this up quickly. There is a ton of detail in the Nether Update for things like weather, particle effects, sound design, animations, and transitions between biomes. Everything should blend more seamlessly. Also piglins can dance.
• Changes to redstone wires. Redstone wires are getting some changes to make them more sensible if maybe breaking them a little bit for older contraptions. This means more visual cues for when redstone wires are powering blocks, and changes to how they power blocks.

Related: A Minecraft: Java Edition snapshot makes some changes to redstone wires