The zygote simply is the fertilized ovum or egg cell or female reproductive cell. It is the result of the process of fertilization of the fusion of the male reproductive cell (gamete) and the female reproductive cell ( gamete). In human, the zygote is the resulted cell of fertilization of the sperm and the ovum. It is diploid or 2N which means it contains two sets of chromosomes; one set from the male and the other from the female.
The gamete is the reproductive cell. There are two types of gametes; male gamete and female gamete. In human, the male gamete is called the sperm and the female gamete is called the ovum. The gamete is the last stage of the reproductive cells to be completely formed.
Fertilization is the fusion of the male gamete and the female gamete to form the zygote. Fertilization may be external as in fish or internal as in human.