Minecraft wiki entry for Development_versions, associated text: Are now available in item form; can be given or summoned.
Similarly to command blocks, its placement, destruction, and modification are restricted to opped players in creative.
Now comes with an integrated GUI and new textures.
Has four modes:
Save - build structure in world, tell the block where it is and it saves it to file.
Load - loads saved structure files, replacing blocks with saved blocks except for structure void blocks.
Corner - for automatic size calculation.
Data - for chest markers and other blocks that require data (default).
Structures are limited to a size of 32 blocks in each direction.
The coordinates used by the blocks are relative to themselves.
When in use, it shows the outline of the structure it relates to in white.
Can rotate and mirror structures before they are placed.
Small red and blue cubes can be displayed in the selection area.
Blue is for air blocks, red is for structure void blocks.
Save and Load structure blocks can be activated by redstone.
The game protects against maps saving arbitrarily massive amounts of data to disk.
Display a name tag above them indicating their mode and structure name.
New NBT tags
powered: Whether or not the structure block is currently being powered with redstone.
showboundingbox: Whether or not the bounding box of the structure is being outlined.
showair: Whether or not air blocks are indicated within the structure.