Quote from power crystals
TIleEntities. TileEntities are magical. You can have up to 16 different TileEntity instance types per block (one per metadata), and each TE can track whatever state it wants to. Moving everything ever into one TileEntity can degrade performance in multiplayer as you'll have to send all that data back and forth constantly, but you can easily shove multiple types of things into one block by having each block metadata value map to a different class of TileEntity. It's actually not very hard to do, especially if your blocks are primarily decorative.
how can you get more information about tile entitys? like how to make em work.