From Wikipedia:

   Galaga is an arcade game with a fixed shooter, developed and published by Namco in Japan and published by Midway in North America in 1981. 

This is a sequel to Galaxian, released in 1979. 

The Galaga gameplay allows the player to control the spaceship, which is located at the bottom of the screen. 

At the beginning of each stage, the area is empty, but over time the enemy aliens will arrive in action, and as soon as all the enemies appear on the screen, they will descend to the ship of the player information of one or more and can either shoot at him or collide with him. During the entire stage, the player can shoot at the enemies, and as soon as all the enemies are defeated, the player will move on to the next stage.

   Galaga is one of the most successful games of the Golden Age of arcade video games. The arcade version was ported to many consoles, and it had several sequels.

   The goal of Galaga - to score as many points, destroying enemies, like insects. 

The player controls the fighter, which can move left and right on the bottom of the playing field. Enemies gather in groups at the top of the screen and then begin to fly to the player, shooting him with bullets and trying to crash into him. In later stages, some enemies even break out of the incoming group to desperately try to crash into the player. 

The game ends when the player's last fighter is lost, either as a result of a collision with the enemy, one of his bullets or as a result of capture.

   Galaga introduces a number of new features compared to its predecessor, Galaxian. Among them - the ability to make more than one shot at a time, counting the "hit/misses ratio" of the player at the end of the game and the bonus "Complex stage", which occurs every few stages, in which a series of enemies fly out of the screen and exit it on a given scheme, without shooting at the player's ship and not trying to crash into it. These stages give a bonus of 10,000 points if the player manages to destroy each enemy, but otherwise - 100 bonus points for each destroyed enemy.

   Another feature of the gameplay, new to Galaga- is the ability of enemies to capture the player's fighter. 

While the player controls only one fighter, the "boss" of Galag (who needs two blows to kill) will periodically try to capture the fighter with the help of a traction beam. 

If successful, the fighter joins the enemy as a companion of the boss Who captured him. 

The captive fighter becomes the enemy, and he can also be shot and destroyed. 

The player can still shoot while captured until his ship "touches" the kidnapper, which can be seen as an opportunity to shoot down the invader before he can successfully capture the player. 

Captive fighters can be released by destroying boss Galaga, towing him while he attacks. 

The freed fighter will then team up with the player who liberated him, offering double the firepower, but with a lack of purpose, twice as much. 

If the player destroys the kidnapper while he is still in service, the captured fighter will not be rescued and will instead fly away after jumping into the water (assuming the player does not destroy him) to appear as a satellite for another at the next stage. Boss Galaga, where he can be saved again.