Can someone explain VX and VX Ace tilesets?

What does that star mean?

Also what is the difference between the various tileset types? (Overworld, etc, A, B)

I only experienced, that the first field allways must be an "*", if it contains your own graphics.
Else every tile is passable, if it's a "O". Or everything is not passable, if it's a "X".

I had that problem, when I made my first "GIAW" project. But basicaly the "*" means the character walks behind that sprite/tile. (\s/)

The * means that your character can walk behind it.

Acolyte here. Too lazy to switch acounts.
A1 - A4 tile = autotiles (grass, fences, etc.)
A5 tile = Regular static tiles, only not a full sheet, and has the same priority level as the other A tiles.
B, C, D, and E = Regular vx style tiles

The default tiles labeled "Overworld", "Interior", "Exterior", and so on, are just little RTP packs that enterbrain included to show off how you can have different tilesets for different maps.