Strength: Measures your character's physical strength. Influences the power of close-combat weapons.
Speed: How fast your character can move. If you have a higher speed than your opponents in a battle, then your turn is before theirs and you can outrun them (also your reflexes are faster than your opponents).
Endurance: Your character's stamina. It represents how long your character can run around and shoot stuff without resting. Can be reduced drastically with tranquilizer darts. If it hits 0, your character loses consciousness.
Toughness: Your character's resilience to wounds. Acts as your character's life bar from wounds.
Intelligence: Your character's intelligence. Affects your character's ability to use complicated technology and weapons.