The gametes, or oocytes and sperm, are the cells types responsible for reproduction. As such, these cells must retain the ability and information needed to produce all possible cell types in the next generation. They are essentially an unbroken chain of life.
Oocytes and sperm are similar in that they both undergo meiosis to produce haploid cells, but the timing and specific details of oogenesis and spermatagenesis differ between males and females. Additionally, each species often has unique features of gametogenesis.
A summary can be found in the section,The structure of the gametes, from Gilbert's Developmental biology here: https://www.ncbi.nlm.nih.gov/books/NBK10005/
Fertilization might seem like it "just happens" but it is actually a complex process process, some aspects of which are still not entirely charactertized.
Review the chapter,The structure of the gametes, from Gilbert's Developmental biology here: https://www.ncbi.nlm.nih.gov/books/NBK10005/