What is sp$3$ hybridization?

What is its use or need? Why is it or how is it different from Molecular orbitals? Or is it same?

Are there only $3$ or $4$ types of hybridization? Or are they most common type? Is sp$3$ hybridization naturally happening or man made?

What is sp$3$ hybridization in silicon and why is it needed?

Why sp$3$ when it has $3$ p orbitals$($ px$,$py$,$pz$)$ does its diagram only show $4$ p orbitals $($ dumbells$)$ when it should show $6?$