I had not heard this term before, but I assume you are referring to https://en.wikipedia.org/wiki/Cyclic_quadrilateral (everything on that page is way beyond the scope of the GMAT).

Some important things to remember when you see a shape inscribed in a circle:

1. The distance from the center of the circle to each corner of the polygon is the same, since each such distance is a radius.

2. The angle measure of any internal angle of the polygon is equal to half of the corresponding central angle. So, if angle ABC has a measure of n degrees, then angle AOC, where O is the center of the circle, has a measure of 2n.

Hope this helps.

Abhi7 wrote:What are the properties of Cyclic quad's that are different from those of normal quad's?

Hi,
gmatboost has already answered your question. Because you are looking for a specific difference, I will just add the interpretation of point 2 posted by gmatboost. Sum of opposite angles of a cyclic quadrilateral is always 180 degrees.