Galaga is the sequel that outgrew its parent. Namco released it in 1981 after Galaxian, and its addition was one of the best ideas in arcade history: the boss aliens can capture your ship with a tractor beam, and if you shoot the captor while your ship is being held, you get it back — flying alongside you as a dual fighter with twice the firepower.
Everything else is precision: a formation of insect-like aliens assembles at the top of the screen, peels off in diving attacks, and you move left and right along the bottom shooting them down. Between stages there are Challenging Stages — bonus rounds where nothing shoots back and a perfect run is worth 10,000 points.
Let a boss Galaga capture your fighter with its tractor beam, then shoot that boss the next time it dives. Your captured ship joins the one you're flying, doubling your firepower — at the cost of a wider target.
A bonus round every few stages where waves fly in set patterns and never shoot at you. Hitting all forty is worth a 10,000-point bonus.
Galaga is the 1981 sequel to Galaxian (1979). It adds the tractor beam and dual fighter, the Challenging Stages, and much more aggressive dive patterns.