orbital hybridization

The process is a purely mathematical technique, brought into operation when experimental results indicate that the bonding in compounds is not accurately reflected by the use of pure atomic orbitals.

1) Hybridization is a mising of atomic orbitals on a central atom to give a set of equivalent hybrid orbitals.

2) Only orbitals which lie closer together in energy can be used in construction of hybrid orbitals.

3) The number of hybrid orbitals always equals to the number of component atomic orbitals.

4) Hybrid orbitals are more effective than s/p/d in overlapping and form stronger bond.

d orbitals are sometimes also involved in hybridization, in the case when a period 3 or above element is the central atom (since elements in period 2 or below do not have low-lying d orbitals for hybridization. As a result, it is impossible for an element in period 2 or below to expand its octet.)

Say for example for PCl5, P is the central atom of the molecule and it forms 5 sigma bonds with 5 chlorine atoms, and hence 5 hybrid orbitals are needed in which the hybridization of P is sp3d.

Bonds are formed when electrons are sharing (basically speaking).
Orbital hybridisation is the so-called mixing of of different orbitals to form bonds and these are mathematical solutions which only a physical chemist can truely understand (I'm only an organic chemist, I can only remember what I learnt in undergrad. years!)
You need to be familiarised with all the orbitals first before going any further:
s-orbital, p-orbital, d-orbital and f-orbital
A molecule is what they look like (ie. 3D molecules) is all because of the mixing of these 3D orbitals. Much like x, y, z axes.
sp3, sp3d and all that are one step further in the discussion - these are to do with LUMO and HOMO (ie. bonding, non-bonding and anti-bonding orbitals), mainly to do with electrons excitation into a different orbital due to the amount of energy put into the system.
It's very hard to explain and understand without those basic concepts. Maybe read up a bit on those and then come back for more discussions.
Hope that helps.