A full changelog can be found on Minecraft.net:

CHANGES IN 18W01A

• Added facing part of /teleport
• Added set_name function of loot tables
• Added minecraft:load function tag, ran once after a (re)load
• Added noon and midnight to /time set
• Crash reports now list what data packs are enabled
• Technical: Changed all custom names (blocks, items, entities, block entities) to translatable text components
• Technical: We're exposing our data generators, so if you can figure out how, you can get a dump of our all blocks/items/commands/etc from the game without opening it up!
• Fixed bug MC-2340 - Redstone torches schedule updates when they should not, causing unreliable timings.
• Fixed bug MC-9194 - A Comparator can lock a Repeater, but the Repeater doesn't look like it is locked
• Fixed bug MC-64836 - Mobs "control" the minecart they are riding
• Fixed bug [...............] x 53

To get snapshots, open your launcher and go to the "launch options" tab. Check the box saying "Enable snapshots" and save. To switch between the snapshot and normal version, you can find a new dropdown menu next to the "Play" button. Back up your world first or run the game on in a different folder (In the "launch options" page).

Please report any and all bugs you find in Minecraft to bugs.mojang.com.

Snapshots can corrupt your world, please backup and/or run them in a different folder from your main worlds. 1.13 will have a lot of experimental snapshots that may break everything and smell funny. This may be one of those snapshots. This is what happens when Mojang are changing all the things! 

Share your thoughts on how 1.13 is shaping up in the comments below!