Backdrops are variant forms of several normal blocks that do not obstruct movement but still retain other properties such as enchantment bonus or water redirection.
They have a whiter, washed out appearance making them distinguishable from foreground blocks (unless already white).
The block itself yeilds its backdrop, and vice versa, so they convert back and forth freely.
A world tile can only hold one block, so it's not possible to have both foreground and background on a tile.
Warning: Backdrops do not prevent in-world replacement by a foreground block, which destroys them.
Backdrops are immune to normal breaking, but can be broken if holding another backdrop block.
Flamable backdrops can also be ignited by an adjacent fire, converting them to items.