The little cracks on your wall can get so big if not quickly fixed, and it

may totally deface your building.
Plaster cracks are caused by the following reasons--
(i) Inadequate use of cement
(ii)Handling of the job by incompetent and impatient masons
(iii) When water is not enough in the mix
(iv)portion of plaster surface is continually exposed to water

To fix, there are different methods you can apply.

First, scrape out the scale or the exposed surface, wash and clean the

surface with water and remove other dirts and growths like algae which 

may affect the bonding strenght

After that depending on how large the surface area, put wire mesh (small

tiny iron) in the surface and apply the mortar to cover the portion, the

wire mesh is to make sure the plaster does not fall off from a large

surface area with thick plastering.

Read more  http://www.buildingcontractorsecrets.com/2008/08/how-to-fix-plaster-cracks-and-causes.html