diploid - complete set of paired chromosomes from each parent (46 chromosomes [humans] )
haploid - single set of chromosomes (23 [humans] )
Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
Sexual reproduction involves the production of gametes by meiosis. These then fuse to form a genetically unique individual.