Mortar is used to bond masonry units together. It doesn't need the strength of concrete; it needs good bonding characteristics. Mortars with a high water-cement ratio bond best. This is the opposite of concrete, where a lower water content makes it stronger and more effective for its purpose. Additives or plasticizers may be added to mortar mixes to increase their flexibility. Lime acts as a plasticizer in Portland-lime cement; in masonry cement, plasticizer is in the premixed product.
Structural concrete is any structural piece made of concrete that has points of support and requires the capacity to hold loads, such as concrete beams, suspended concrete slabs, or concrete pilings. Plain concrete is any structure supported by the ground, such as walkways, driveways, slabs, and spread footings to support masonry walls. Plain concrete is often reinforced with steel to increase its tensile strength against temperature fluctuations that threaten its structural integrity.
There are two kinds of mortar used in construction: Portland-lime and masonry cement. Portland-lime mortars are mixtures of Portland cement, hydrated lime or lime putty, sand and water. The bonding ability of Portland-lime mortar isn't as high as its strength; it's used for heavy stone construction where a lot of weight will be carried. Masonry cement is a more-common product; it consists of premixed cement materials, including plasticizer, mixed with sand and water. It's used for general brick- and clay-unit masonry, and for setting ceramic tile.
Concrete mix is made up of Portland cement, water and aggregates. Portland cement and water are active ingredients; in combination they form a paste that binds the aggregate together. The size of the aggregate used in the mix varies according to the engineering specifications and local availability. A key difference between concrete mix and mortar mix is that coarse aggregate--gravel--is used in concrete mix, and only fine aggregate--sand--is used in mortar mix.