What Is Mortar Used For ?

Mortar is actually the 'mix' which binds and bonds bricks together in a wall, providing the wall with strength and durability.
Mortar is made from cement, sand and hydrated lime (or liquid plasticiser) which is then mixed with water.
The standard mix is on part Portland cement, 1 part hydrated lime or plasticiser to 6 parts sand.  The mixture is made by adding first the cement t the sand, then add in the lime.  A small well should be made in the mix and water added until the mixture is stiff but pliable and can fall off the shovel cleanly.
Mortar goes off within about two hours of being mixed and so it is important to mix only enough for use within that two hours.  It is best to mix only a little, then a little more etc.
For small jobs it is worth buying ready mixed bags of mortar, but if you are laying more than 150 bricks it is possibly more cost effective to buy the materials from a builder's merchant/DIY store.