The first hybrid orbitals we are going to look at are the sp3 hybrids. They get their name from the fact that they are a combination of one s atomic orbital and three p shaped atomic orbitals. As the original orbitals are simply mathematical functions (that describe the wave that is the electron), these new hybrids are simply new mathematical functions that are combinations of the old ones (the old ones added together in various ratios). When making hybrid orbitals, you make as many hybrid orbitals as the number of atomic orbitals you start with. Thus one s and three p's is four total atomic orbitals (AO). This makes four sp3 orbitals. These orbitals are arranged in space such that they point at the corners of a tetrahedron. That is they have a tetrahedral geometry. Thus, we would say that any atomic center that has an electronic geometry that is tetrahedral is sp3 hybridized.
The four sp3 orbitals all have the same energy and they all lie somewhere between the energy of the s and p orbitals that were used in the hybridization.