What is the difference between didentate ligand and ambidentate ligand?

Didentate or Bidentate ligands are the logands which makes bonds with the metal ion in two places. It make two bonds with two different portion of its structure. Ambidentate ligands are the types of ligands which have the possibility of forming bond with central metal ion from two sites but they only form one. This results into two different kinds of molecules with the same molecular formula.

a bidentate is one which combines from 2 different sites simultaneously....but ambidentae is one which can combine from 2 sites but at a time only one site can combine(ambidentate ar unidentate having 2 sites available out of wich one combines at a time)...

Bidentate ligand

A bidentate ligand has two points at which it can attach to the central atom. One example of such a ligand is:
oxalate ion

The two single-bonded oxygen atoms can each donate electrons to a central atom.
Three oxalate ions can coordinate a single central atom, giving an octahedral complex.

Ambidentate ligand
Ligands with more than one potential donor atom are known as ambidentate, such as the thiocyanate ion, NCS-, which can bind to the metal center with either the nitrogen or sulphur atoms. Examples of ambidentate ligands include NO^2-/ONO^- (O and N), and SO₃^2-/OSO₂^2- (O and S), where the first named atom refers to that which is bonded to the metal center.

The existence of ambidentate ligands means that there can be linkage isomerism: [Co(NO₂)(NH₃)₅]²⁺ exists in a red form and a yellow form. The red form has the NO^2- group bound by oxygen, when the ligand is known as the nitrito group, and the yellow form has the NO^2- group bound by nitrogen, when the ligand is known as the nitro group.