What is the spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

(N/A) spectrochemical series is an arrangement of common ligands in the increasing order of their crystal-field splitting energy $(CFSE)$ values.
Ligands on the right-hand side $(R.H.S.)$ of the series are strong field ligands,while those on the left-hand side $(L.H.S.)$ are weak field ligands.
Strong field ligands cause greater splitting of the $d$-orbitals compared to weak field ligands.
The series is: $I^{-} < Br^{-} < S^{2-} < SCN^{-} < Cl^{-} < N_{3}^{-} < F^{-} < OH^{-} < C_{2}O_{4}^{2-} \approx H_{2}O < NCS^{-} < H^{-} < CN^{-} < NH_{3} < en \approx SO_{3}^{2-} < NO_{2}^{-} < phen < CO$.