In asexual reproduction, organisms are basically being copied. There is only one set of DNA that is being copied between generations, so all offspring have the same DNA (unless there is a mutation).
In sexual reproduction, since each gamete contains half the genetic information for the organism being produced, there are many combinations possible, each of which would produce a genetically unique offspring.
(Assuming you know the concept of crossing over ) In Asexual reproduction all the progeny resembles the parents because there is no crossing over in asexual reproduction.
But since Crossing Over is present in sexual reproduction , there is variability among organisms.