Answer

Static friction is the friction between the two bodies before the body starts moving. Therefore, it is the friction which acts between the two surfaces when there is no relative motion. For example, when you are pushing a box, initially, you are not able to move it because the force you are applying is not enough to overcome the force due to static friction. But when you try it with more force, it starts moving. So the force you are now applying to keep the box moving with the same speed is a measure of sliding friction. It is the friction between two objects, when they are sliding against each other.

In sliding friction the motion has been started and inertia of the rest has been overcome. With the start of the motion,  irregularities of one surface have little time to get locked again into the irregularities of the other surface. 
This is why static friction is always more than the sliding friction in which motion has been started.
 

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