From Artifact Wiki
Artifact is a digital card game, announced to release in 2018.
- No singleplayer campaigns
- "An AI will teach the player how to play" (A tutorial, then?)
- Known Heroes
- Different modes of play, such as draft or sealed deck.
- Shopping for items is a different game phase.
- 4 types of heroes, those being black, red, green, and blue 
- Black - seems to be the "assassin" type, can influence other boards. Specializes in Gold generation, good single target removal, and dealing tower attack damage.
- Blue - the "spellcaster" type, said to be focused on scaling into the late game.
- Red - The "Strengh" type, said to be good for their stats.
- Green - The "Support" type.
- Spells are divided into the same categories, and can only be used if a hero of said category is present in the chosen lane. (multi-color cards aren't mentioned in any way, but might still exist) 
- Each turn, 2 melee creeps spawn for each player in random lanes. 
- Win condition is to destroy 2 towers or to destroy the enemy ancient after destroying any tower. 
- On the first turn, you evenly deploy 3 heroes on 3 lanes (meaning that 2 heroes sit out the first turn) 
- Killed heroes "respawn" and can be redeployed after being gone for 1 turn, unless the hero has the ability that allows for instant respawn. 
- Killed heroes keep their equipment through death. 
- Each lane has its own mana pool, starting at 3 and increasing by 1 each turn. Some cards can affect the mana gain (ramp it up, as in the given example)
- Each time a player plays a card in lane, initiative goes to their opponent, and after both finish playing cards in that lane, combat begins. Initiative then carries over to the next lane. 
- Destroying enemy cards gives gold to spend in the shopping phase. 
- Heroes have 3 equipment slots, for weapons, armour, and accessories (giving damage, armour, health or other effects respectively) 
- "A hundred" units can be on one lane at the same time. While not clear if that's the real limit, it's said that you can scroll the lane with your mousewheel to see off-screen characters.
- Each round you draw 2 cards. 
- Towers have 40 health, while the Ancients have 80 health.
- There is no limit to how many cards you can have in your hand.
- There is no limit to how much mana can you have (it will keep increasing by 1 point each turn indefinitely)
- Negative amounts of armor will increase the amount of damage that you take.
- So far, the card count sits at 280 cards and 44 heroes, according to Geoff Keighley
- Each desk has 40 cards, 5 of which are heroes. 
- There is no maximum to the amount of cards a player can hold at any given time.
- Cards can have pro player signatures on them.
- Up to 3 of the same card can be in a deck.
- There at least 3 damage types, including:
- Normal Damage
- Siege Damage: Tower damage that can't be blocked.
- Piercing Damage: seen on Luna's Lucent Beam ability.
- Buying card packs is confirmed.
- Selling cards on the marketplace is Confirmed
- Trading was mentioned.
- Power level of the cards doesn't coincide with rarity.
- "Common cards will cost pennies"
- Opening card packs is said to be a spectate-able feature
- Workshop integration is confirmed, which can mean all kinds of different user created content. (Nothing is specified, but it can mean cards, skins, music packs,card backs, and other things?)
- Artifact is not one of the valve's three fully-fledged VR titles, but it is unknown if VR support is available.
- Artifact does not reuse assets from DOTA, as you can see from multiple gameplay screenshots.
- Artifact is confirmed to be a Source 2 title. 
- Artifact is announced to come to iOS and Android in mid-2019. 
- Replays and player stats will be available (source?)
From Erik Robsons tweets
- The cards are expected to have 3D models, though the exact way the system will work is not known (whether it's a 3d representation of the card, a 3d model depicting a hero, a 3d model inside the card, or anything in between. Reading the source tweet is encouraged)
- Status effects are mentioned (being sprites layered on top of a 3D card model)
From VP interviews
- Virtus.Pro players are one of the first in the incoming alpha-testing, organization is in good contact with Valve
- Game is "Already playable" at Valve HQ
- Several people from WESA have tried the game, are excited
- Opening card packs will feed the Prize pools, much like compendiums do in Dota 2.
- A tournament with a $1 Million prize is scheduled for first quarter of 2019.
- Automated tournaments for all skill levels - probably similar to how StarCraft 2 handles it.
- SirActionSlacks - A well known DOTA 2 personality, is in contact with Valve and confirmed to be under an NDA.
- Day9 (Day9TV) - A famous youtuber and streamer, known for his StarCraft & Hearthstone content. He was invited to the TI7 and was the one to share the info about the gameplay right after the teaser.
- SirActionSlacks said in an interview, that took place around August 10, 2017 that he was shown Artifact "a couple months ago" He, and, presumably, other people who were shown Artifact, are under a NDA.
- The Steam AppID is very likely to be 583950
- Brandon Reinhart stated in a reddit thread regarding this AppID, that Valve pushed an update hiding playercounts of unreleased AppIDs, which is not an official confirmation of this App being Artifact, but can be considered a rather blatant hint.
- Peak playercounts for this AppID were said to be ~27 players at the same time, and it seems like multiple community figures are very involved in testing the game.
- The artifact symbol seems to be used for the "end turn" button, or something similar to that.