What is locus Please explain with diagrams??

A locus, in mathematics, can be defined as a set of points which share a common property or satisfy a certain condition. Circle is the best example which is the locus of points equidistant from a given point.

Example: A Circle is "the locus of points on a plane that are a certain distance from a central point". As shown below, just a few points start to look like a circle, but if you collect ALL the points, you will actually have a circle.

or else...see the following eg :
 for example..

As seen by the headlights (and taillights) in the picture on the top, a locus of points (the headlights or taillights) is the path traced out by the moving points under given conditions (following the road).

hope it helps..