As stated at Chemicool.com, the 2s orbital and the three 2p orbitals hybridize to form a new hybrid orbital, called sp³, which is capable of holding up to eight electrons.
Hybridization happens when atomic orbitals mix to form new atomic orbitals. The new orbitals have the same total electron capacity as the old ones. The properties and energies of the new, hybridized orbitals are an 'average' of the original unhybridized orbitals.
The sp³ hybrid orbital, which is the sum of the original 2s and 2p orbitals, looks something like this:
Hybridization via kwout
sp³ hybrid orbitals have a tetrahedral shape - each orbital makes an angle of 109.5° with the others. This angle maximizes the distance between the orbitals, which is natural given that the electrons in each orbital repel one another.
The shape of molecules like methane, CH₄, with bond angles of 109.5°, is consistent with sp³ hybridization of carbon atoms.
Definition of hybridization - Chemistry Dictionary via kwout
Here is an energy level diagram showing how electron energies change in hybridization:
Definition of hybridization - Chemistry Dictionary via kwout
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