Meiosis

Sexual reproduction required meiosis, a process in which a parent cell divides to produce cells with half the genetic material of the parent. A diploid (2n) parent cell, for example, divides to make four haploid (n) gamete from two individuals then combine to produce a diploid zygote. The offspring resulting form sexual reproduction is genetically different from both parents. (From Sadava, et. al., Life: The Science of Biology, 9th Edition, 2010). 

(Image taken from:http://www.hartnell.edu/tutorials/biology/meiosis.html)

Important points to note about meiosis

1. Crossing (also called genetic recombination) over occurs in Propahse 1. This allows exchange of genetic materials, that would results in genetic variation.  Every human is different from another because of this process. It occurs at randomly. Important to note that crossing over does not occur on X and Y chromosomes, because they are not homologous pairs.

2. Occurs in two stages

3. Sister chromatids do not seperate during first stage of meiosis

4. Four daughter cells are produced having haploid (half or n) number of chromosomes. These cells are genetically different from the parent cell.

5. The haploid (23 chromosomes) gametes formed at end of meiosis will later fuse with another gamete of opposite gender, forming a diploid zygote (with 46 chromosomes)

Here is a useful link an animation showing meiosis
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html