This game is a shoot-em-up! The aliens are invading (from the right) and you have to shoot them while avoiding them. Your ship starts on the left. You use the cursor keys to move and the spacebar to fire. Cloud variables are used to keep a high score. Space Swarm is the last big project in my book Scratch Programming in Easy Steps.
This is an example from the book Scratch Programming in Easy Steps by Sean McManus. More info here: http://www.sean.co.uk/books/scratch-programming-in-easy-steps/index.shtm
