At its core, Warhammer 40,000 is a tabletop wargame played with miniatures. The original game was released in 1987 but has since spun out various other forms of entertainment centering on the same universe; books, films, video games, board games, etc.

Painting the miniatures is an integral part of Warhammer 40k. Most tournaments don't event allow players to compete with an unpainted army. Also, a significant portion of Warhammer 40k fans collect and paint miniatures but never play with them.

The exact game rules differ from edition to edition, and we are currently playing on 9th edition as of 2022, but I'll touch the more general concepts there.

Army

Choose your preferred army by any of the following criteria, or a mixture of these:

1. look - general aesthetics of the race, how do they look like
2. lore - what is the story behind the group or race
3. power - some people prefer playing the most overpowered army at any given time
4. playstyle - close-combat, ranged, magic, durable, a lot of small units
5. price - some armies are more expensive to build as they contain more miniatures
6. allies - which other armies can you mix with this army

Model:

• Each figurine on the battlefield is considered simply as "a model".
• Units usually consist of multiple models so, in a sense, each model is one health point if the unit statistics state that "Wounds (W)" is 1.

Unit:

• Your armies are composed of units. Models move and fight together as units.
• Unit Coherency: Models under the same unit must stay and fight together when not actively performing a move action. Each model must be within 2 inches horizontally and 6 inches vertically from another model in the unit, forming an unbreakable chain.

Each unit has statistics.

• Move (M): how far can each model in the unit move
• Weapon Skill (WS): roll to hit in close-combat
• Ballistic Skill (BS): roll to hit in ranged combat
• Strength (S): determines damage of your close-combat attacks
• Toughness (T): can allow resisting being wounded
• Wounds (W): how many wounds it takes to kill the unit
• Attacks (A): how many dice you roll in the close-combat or fight phases
• Leadership (Ld): unit's will or determination to stay in the battle
• Save (Sv): determines what you get to roll in order to avoid being wounded

All units in an army must have at least one faction keyword in common. For example, Tyranids have TYRANID and <HIVE FLEET> keywords. Genestealer Cultists have TYRANID and CULT keywords. So Tyranids and Cultists can be in a same army. IMPERIUM is the most common keyword.

Battle-forged army needs to meet the following criteria:

• Army consists of units that have at least one faction keyword in common.
• Detachment must contain units which share the same faction keyword, excluding CHAOS, IMPERIUM, AELDARI, YNNARI or TYRANIDS. This means you can have Genestealer Cultists and Tyranids in the same army, but they need to be in separate detachments.
• Each model has a point cost. Keep that sum under the specified point limit.

Detachment:

• A detachment is a group of units that follow a specific template.
• A battle-forged army is composed of detachments.
• Each unit can only belong into a single detachment.
• Each unit has a specific battlefield role: HQ ("leader"), Troop, Elite, Fast Attack, Heavy Support, Dedicated Transport, Flyer, Fortification, Lord of War.
• Detachment templates list minimum and maximum number of units for each battlefield role.
• Usually, you can take 1 transport for each battlefield role your detachment fulfills. It reads on the template.
• Each unit has a "power level", it's an alternative way of limiting armies. Power level is rarely used in competitive as it doesn't count equipment. Point costs for units and equipment are listed at the back of the rulebooks.
• Use https://www.battlescribe.net/ to build your rosters. It helps checking if your detachments and armies are valid.

Matched Play

When you have a battle-forged army, you gain access to command points (CP).

• Your army earns CP based how "efficient" they would be at the logistics of war, but all battle-forged armies start with 3 CP. Adding certain unique units and using certain detachment templates gives you additional CP.
• You use CP to activate stratagems. The most common are 1CP to re-roll a dice, 2CP to get free counter-attack against a charge and 2CP to automatically pass a morale test but different missions and factions have unique stratagems

Read the rules of each organized event before attending.

• A battle-forged army is frequently required.
• What is the army point limit? Usually 1000-3000 points. This is clear indicator how long each game will take.
• How many reinforcements are allowed? You usually have separate reinforcement points for summoned units, just subtract when they are summoned.
• It is common to limit 2-5 detachments per army. This limits that you can't just include units without thinking about detachment templates; individual units form singular detachments if they don't belong in any detachment.
• It is common to limit how many times each datasheet can be included, e.g. 2-4 times per army. This limits building your army with only one specific unit type.
• You should assume that an unpainted army can't attend, but read if there are more specific rules on this e.g. "bases must be painted".