A block is a type of building material. Stone and wood can be shaped into blocks at a mason's workshop or carpenter's workshop. Glass can be formed as blocks at a glass furnace, ceramic blocks (labeled as "bricks") can be made at a kiln, and metal blocks can be made at a forge from a single metal bar (or, in the case of adamantine, from four wafers). Blocks are mostly equal to bars, but are used differently and counts as a different type of items. Blocks do not have quality levels.
A single block is required in order to build a well, screw pump, or ashery. Blocks can also be used in place of raw stone or wood in roads, bridges, workshops and constructions (but not in items like crafts or furniture). Bridges, supports, roads, and constructions built from rocks are called "rough" while the same built from blocks are not given that adjective. Stone blocks are sometimes used as materials during strange moods.
 Blocks vs. rocks
The clearest distinction between a building built with a boulder and building built with a stone block is that more buildings can be built of blocks, as each stone produces four blocks. Note that this does not apply to wood, glass, or ceramic blocks/bricks, which are only made in sets of 1; metal blocks are made in sets of 1, but since you get 4 bars at a time from smelting ore, the end result is equivalent to stone.
Blocks have less volume and thus weighs much less than raw forms of construction materials - stone blocks weigh only 6% as much as a boulder, and wooden blocks weigh 12% as much as the log from which they are carved. This weight difference can dramatically reduce hauling time, especially if you are using large amounts of materials. Forming metal bars into blocks makes no difference in regards to weight, though it does simplify the tracking of resources - if you use metal bars to build workshops and constructions, your stocks screen will still list them in the summary view, potentially misleading you into believing you have more available bars than you actually have.
Building a bridge out of blocks instead of rocks also cuts the building time by two thirds, independent of clearing and hauling time. This difference in building time presumably extends to other buildings and constructions where you can choose between blocks and rocks.
Blocks are more valuable than rocks. Blocks have a base value of 5, compared to the raw material value of 3 for stone, wood or glass. Metal bars already have a base value of 5.
Metal ores and economic stone will still produce multiple stone blocks if cut in a mason's workshop. However, this breaks use for any purpose other than building and strange moods, such as reactions: blocks of flux cannot be used for making pig iron or steel, nor can blocks of ore be smelted into metal (without modding reactions). Note that "filler" blocks of raw adamantine are excellent for increasing artifact value without wasting too much adamantine on legendary doors, tables, etc.