The blocks I recognize the most are the motion blocks as well as some of the repitition blocks and event blocks. What is unfamiliar is the purple looks blocks as well as the operator blocks.
The scratch screen is 480x360px.
The center of the scratch screen is at (0,0) on the coordinate plane.
The unit of measurement on the scratch screen is pixels.
You can have an infinite amount of sprites in your project, but only one stage. The stage could have multiple backdrops, but it wouldn't make sense to have multiple stages for a single "play" or animation.