Technically, the term "clone" means that the process must produce genetically identical offspring. This means that the cloned animal, like Dolly, is an exact copy of the single animal that donated the entirety of its genes. In contrast, the offspring of sexually-initiated reproduction, which so far includes all of us, gains half of its genes from a mother and half from a father.
In humans, this union of the father's and mother's genetic material takes place during fertilization, when a sperm cell combines with an egg cell (or oocyte). To aid fertilization, the egg cell contains proteins (called cytoplasmic proteins) that direct genes in both the egg and sperm cells to begin the process of embryo creation.
In normal fertilization, the egg cell's cytoplasmic proteins turn off adult genes being expressed in the sperm and reprogram other genes into embryonic development. This crucial function of the egg is what enables cloning.
After fertilization, rapid cell division, known as cleavage, takes place. By the end of the third day, sixteen cells are present. In the fourth or fifth day following fertilization, continuing cleavage has caused the egg to develop into a hollow sphere of cells called a blastocyst. On about the sixth day, the blastocyst attaches to the wall of the uterus in a process called implantation.The developing embryo becomes a fetus during about the eighth week, and the pregnancy continues toward birth (Tortura and Grabowski, 199).
Sometimes a fertilized ovum splits in an early stage of development. This produces identical, or monozygotic, twins that are genetically identical to each other, but not to either parent. Each sibling has genes from both father and mother. Because of this, they are genetically identical to each other and are always of the same sex, but, technically, are not clones.