In the film's universe, 'Extraction' is the standard corporate espionage technique of entering a target's dream to steal secrets. 'Inception' is the much more difficult and theoretically impossible task of planting an idea into a target's subconscious so deeply that they believe it is their own original thought.

Dreams are structured in nested levels. As you go deeper, time moves slower. The general rule is that 5 minutes in reality equals 1 hour in the dream. This compounds with each level: Level 1 (Rainy City) is days, Level 2 (Hotel) is weeks, Level 3 (Snow Fortress) is months, and Limbo can feel like decades, allowing for a lifetime to pass in mere minutes of reality.

Limbo is unconstructed dream space, a realm of pure subconscious. It is dangerous because if a dreamer dies while heavily sedated, they drop into Limbo instead of waking up. There, the mind can become trapped, losing its grasp on reality and forgetting that the world isn't real, leaving the dreamer in a coma-like state in the real world.

The spinning top was originally Mal's totem. In the dream world, it spins indefinitely without toppling. In reality, physics applies, and it eventually falls. Cobb uses it to verify his reality. The film's ambiguous ending cuts to black while the top is still spinning but wobbling, sparking endless debate about whether Cobb is awake or dreaming.

Mal is not the real person, but a 'Shadow'—a projection of Cobb's subconscious guilt. The real Mal committed suicide because Cobb performed inception on her, making her question reality. His guilt manifests as a violent, sabotaging force in his dreams, constantly trying to ruin his missions and drag him into Limbo to be with her.

A 'Kick' is a physical sensation of falling or impact used to wake a dreamer from a sedated sleep. In multi-level dreams, kicks must be synchronized across levels (e.g., the van hitting the water, the elevator dropping, the explosion in the fortress) to pull the team out of the nested dreams simultaneously.

While the Architect can manipulate physics (like folding cities), making too many unrealistic changes alerts the dreamer's subconscious. The subconscious defends itself with 'projections' (the people in the dream) which will attack the intruders like white blood cells attacking a virus. Keeping the dream grounded keeps the projections calm.

Ariadne is the Architect. Her job is to design the dream levels—the mazes, buildings, and landscapes—that the dream will take place in. She teaches Cobb that he cannot build the dreams himself because his subconscious (Mal) would know the layout and sabotage them. She acts as the team's anchor to reason and Cobb's therapist.

The team was hired by Saito to perform inception on Robert Fischer, the heir to a massive energy conglomerate. The goal was to plant the idea that he should dissolve his father's empire, not out of destruction, but to be his own man. This would prevent a global energy monopoly, benefiting Saito's interests.

This is a popular theory. Some evidence suggests the entire film might be Cobb's dream (e.g., his wedding ring only appears in dream scenes, faceless children). However, Nolan has stated the ambiguity is intentional, emphasizing that Cobb walking away from the totem at the end signifies he has chosen his reality, regardless of its physical truth.