Super Mario Galaxy used the transforming platforms originally found in 128, and changed them, so they could be planets Mario could walk on with very little performance issues. Mario flies from planet to planet, collecting coins and star bits, or even defeating enemies along the way. Along the way, he meets Rosalina, the cosmic... Goddess? Normal Person? I mean, she does [SPOILERS!!] at the end of the game... so I mean... maybe? Anyway, we meet Rosalina, a cosmic guide, to help us on our way, and save the princess, as well as the Lumas, small star creatures, that transform into launch stars, and so many more characters found throughout the galaxies.
The galaxies (or levels, as most games would call them) are wonderfully creative. There are over 42 of them, each feeling incredibly distinct, and only 1 or 2 sharing the same theme. Within each galaxy, there are about 3 missions + a bonus star that you have to complete to gain a Power Star, the main collectable in the game. There are 121 Stars that you need to 100% the game, but you'll only need 60 of them to beat Bowser and save Princess Peach.
Super Mario Galaxy is a game that never gets old. Its charming, lovely, fun, and just a well-balanced game. The music, the level design, the plot, everything, is just a lovely experience. If you own a Nintendo Switch, Wii or Wii U, please play this game. It is 100% a 10/10 game.