Variation is the difference found among the offsprings of the same parents and the difference of the offspring with their parents.
Living things show two types of reproduction, asexual and sexual mode of reproduction. Organisms produced as a result of asexual reproduction look alike because they are produced from a single parent. There will be very small differences between them, generated due to small inaccuracies in DNA copying. When a bacterium undergoes binary fission, it divides into two daughter bacteria with equally distributed genetic material. All the daughter bacteria show similarity in their structure and function.
In case of sexual reproduction, two parents are involved. The offspring will acquire haploid set of chromosome from both the parents. This brings variation in the character of the offspring and a great difference exists among the parents and their offsprings. You can take your classroom as an example, where all the students will not look alike even thought they all belong to the same species. Great variation is acquired by any living organism during sexual reproduction.