Galaxian followed Space Invaders by a year, and from the modern-day perspective it feels like exactly what it is: a bridge between Space Invaders and Galaga. Like Space Invaders the game features a single screen and you have to shoot down enemy aliens. Galaxian innovates in many areas, most notably by having the enemies drop down individually instead of en masse to shoot you. There are four different types of enemies worth different point values, and enemies are worth more when you shoot them as they're attacking vs. while they're in formation.
Since Galaga is so well known, it's virtually impossible to compare Galaxian to it. As with many sequels, Galaga really perfects the template presented in Galaxian. You're limited to only one bullet onscreen at a time (a restriction that the enemies don't have, which seems kind of unfair), and so the game requires a level of precision that Galaga doesn't. Galaga's graphics are much better, and the addition of numerous features, including enemies flying onto the screen, tractor beams for the "boss" enemies that provide a fun ship-capturing and recovery mechanic, and bonus challenge stages every few stages, really elevate the game to a higher level.
Overall Galaxian was an entertaining look at the path to Galaga, but not too memorable otherwise. I played the version of the game included in Namco Museum Megamix on Wii, which also includes Galaga's successor, Gaplus, and I'm looking forward to finding out more about that game and the others in that collection before too long.