Object Sets consist of:

object definitions
  Which and how many blocks an object is made of, where to place them etc.

tsa data
  The way the blocks are constructed out of the tile data (since tiles can be a part of multiple "blocks")

graphic data
  Tile graphics somewhat specific to the set (not necessarily unique); 128 tiles (?)

common data
  Tile graphics shared by multiple sets; 128 tiles (?)

palette data
  Palette groups to color the blocks

There are 16 object sets in SMB3.

0x0 Overworld
0x1 Plain
0x2 Fortress
0x3 Hills
0x4 Sky
0x5 Piranha Plant?
0x6 Water
0x7 Mushroom?
0x8 Pipe
0x9 Desert
0xA Ship
0xB Giant
0xC Ice
0xD Cloud
0xE Underground
0xF Spade Bonus Game