Sperm cells are produced in the male sex organs or gonads known as testes.
These are located in the scrotum, a pouch that is outside of the body cavity. The slightly cooler temperature of the scrotum provides optimal conditions for sperm production.
Sperm cells are produced by meiotic cell division in the testes. This is a reduction division which produces four haploid cells called sperm. The original testis cell that undergoes meiotic division is diploid and contains 46 chromosomes. In order to function as a sex cell, its chromosome number must be reduced to the haploid amount, which is 23 chromosomes.
At the end of meiosis, each testis cell gives rise to four haploid sperm cells that contain unique combinations of genes due to the processes of crossing over and independent assortment. When a sperm nucleus that is haploid fuses with an egg nucleus--also haploid--the resulting zygote or fertilized egg is diploid again. This occurs during the process of fertilization.
To conclude, male gonads called testes produce sperm cells for sexual reproduction.
I have included a link to a slide show showing the entire male reproductive system.
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