What is the effective depth of a beam?

Could you please explain to me, in layman's terms, what exactly is meant by the 'effective depth' of a beam in the context of structural engineering? I'm particularly interested in understanding how it differs from the beam's overall depth and what factors influence its calculation. Additionally, how does the effective depth affect the beam's strength and stiffness? Could you provide any real-world examples or scenarios where the effective depth of a beam might be particularly significant?