Mortar is a workable paste of cement, sand and water used to bind bricks, blocks and stones together and to fill the joints between them. Unlike concrete, mortar contains no coarse aggregate — only fine aggregate (sand) and binder — which makes it smooth enough to spread in thin, even joints.
Its job is to bond the masonry units, distribute load evenly between them, and seal the gaps against water and air. Common mix ratios are 1:4 to 1:6 (cement:sand): richer mixes for load-bearing and external walls, leaner mixes for ordinary internal work. A useful principle is that mortar should be slightly weaker than the units it joins, so that any cracking happens in the replaceable joint rather than through the bricks.
The same mortar is used for plastering. Because mortar is finer than concrete with smaller voids, it bulks up by about 33% from wet to dry — the 1.33 dry-volume factor — rather than the 1.54 used for concrete.