A Deck, in the common terminology, is all the cards needed for play, or a set of cards that work together to execute a strategy to win, as they constitute the resources that a player can call upon to battle their opponents in any given game.
In The Horus Heresy: Legions, the Deck could be called the "motor" of the game, and the "true" starting point of play.
Before starting any type of match, in any type of Battle Mode, all Players will have to construct a Deck (of 30 Cards plus a Warlord of any chosen Faction) to fight with. How well this Deck is formed is fundamental for the competitive scene, and even in PVE.
Beside the "special" Battle Modes, all Decks are constructed with a max of 2 of the same Card (for Common, Rares and Epics) and 1 for Legendaries, of Troops and Tactics (or only one type if so preferred). You'll have the option of (only) putting cards from the Warlord Faction, or from the Neutral Factions (Imperial Army, Mechanicum and Chaos).
While the skill one possess when playing will determinate the outcome of a match, is in the Deck building aspect when Players are truly "tested" for their insight on the game. Even the best Player in the world would hardly win any battle with a poorly constructed Deck.
On that regard, the opposite is also true. A Player might use a Community Deck (see below) that has a "high" win rate, but if it doesn't understand how is played (the main win condition) or why is played (the target focus), then they will struggle to execute it properly.
Knowing which cards to put in, how well they synergize with each other, how well the Energy Curve is, how certain the win condition is, how "fast" can it be reached (or "slow" if that's preferred), the Archetype idea to use, and many more aspects when building it, will consistently make you win Battles (as the better the Deck a Player builds, the better that Player becomes).
Types of Decks[]
While all Decks function the same when in combat, how they're build, or the objective of such, change depending on the Battle Mode. These are:
● Standard Deck[]
The "main" or "normal" type, this is the Deck you'll use when playing in Standard Battles (and the result will count towards your Standard rating).

If you place "Premium Cards" in it, you can see the difference by the golden glow of each (you can have 1 normal and 1 premium if available, or place the max possible).

You can also play in Practice or Friendly with these Decks (can be against Standard or Legacy Decks).
● Legacy Deck[]
If you place any Legacy Card when building a Deck (or use a Legacy Warlord), you'll play in the Legacy side of Battles (and the result will count towards your Legacy rating).

Just by having 1 card of it (they will appear with that purple background), any match, of the Ranking Battles, will be against other Players that also uses Legacy Cards.
You can also play in Practice or Friendly with these Decks (can be against Standard or Legacy Decks).
● Event Deck[]
This is the Deck you'll use when playing in Events (and the result will only count towards your Lodge points for that Event duration).

These Decks differ when building them, as you choose a Warlord and 3 cards from 3 different options (generally two are from the Warlord Faction and one from the Neutral ones) 10 times.
Because the cards, and the rarity, of those cards offered varies so much (you could have 2 of the same Legendary from the start, something that can't be done in regular play), it will be hard for a Deck to be replicated (by you or any other Player), making them very singular each run.
While, likelier than not, Standard Cards will appear, there's a chance some Legacy ones do so.
● Campaign Deck[]
Campaign Decks are very similar to Event Decks, as you build it from 3 options with 3 cards each, but instead of doing it so from the beginning, you start with 15 pre-set cards and you "unlock" the options as you defeat enemies Warlords (one each at a time), till you have a 30 cards Deck.

At three victories, or after the sixth if you restart the run and the Epic Boss has been reached, you'll get a "power up" to choose and add to your Deck (from 3 options). This "extra card" (with the red background), gives you different passive abilities (extra life, extra damage, lower amount of Energy the enemy have, etc.) and gets played as the match begins.
Campaigns can only be played (currently) with not reworked Factions, so the amount of available ones will be becoming shorter as time goes on.
● Starter Deck[]
These are pre-constructed Decks that can be bought with real money from the Shop (or offered to when finishing a Campaign), and you can use it to play in Standard Battles.

They give a Warlord (generally a Rare one), a unique Card Back, and a mix of cards of the Warlord Faction and Neutral ones (Commons and Rares, with one guarantee Epic).
You can replicate these Decks if you earn the same cards by yourself, but the idea of them is to help new Player start playing right away (or to build their Collection).
Notes[]
For a more comprehensive, community recommended, guide on how to build Decks or play them, or to get more information on topics mentioned, click in the links below for:
- "Deck Building Guide" -for the basics on how to build Decks.
- "Buyers Guide" -for recommendations on what Cards to buy.
- "New Player Guide" -for when starting out.
- "Advance Guide" -for more complex aspects of the game.
- "Archetype" -for the different types of Decks (objective wise).
- "Resources" -for Energy related content (like Curve).
- "Community Decks" -for a list of Decks made by the game's community.
- "Standard and Legacy Play" -for the differences between the two.