1. Be in outerspace or have a monotonous background,
2. Scroll from top to bottom,
3. shoot crap from a craft/thing,
4. have the bad guys fly at you in that stupid repetitious pattern from the top right or top left, or top right and left at the same time.
Here is the list of games like Galaga, or we can also say like 1942 since they are both the same idea:
1942
1943
Alpha Mission
Dragon Spirit
Dino Riki
Guardian Legend
Gun Nac
Image Fight
Legendary Wings
Sky Shark
Starship Hector
Star Force
Star Soldier
Terra Cresta
ThunderBirds
Tiger Heli
Twin Cobra
Twin Eagle
Xevious
Zanac
Honorable Mentions (Usually hybridizing Galaga and Defender II, by having the ships move horizontally across the screen instead of vertically.):
Air Fortress
Double Strike
Gyruss
Last Starfighter
Lifeforce
SCAT
Section Z
Silver Surfer
Sqoon