Templates is an API for blocks which can change their appearance to those of other blocks. Currently, plain slopes (crafted with five bamboo and one string) are the only built-in template blocks.
Template blocks can be placed in the world, then right-clicked with a full-size block to set the textures for the template. Template blocks will inherit light and redstone values from the blocks they're given, or they can have light or redstone output added to any given block by right-clicking the template with glowstone dust or a redstone torch, respectively.