What is the spectral classification of stars?

The spectral classification of stars is based on the characteristics present in the spectrum of stars.
The modern series of spectral types is O, B, A, F, G, K, M, and has recently been extended towards lower temperatures with two new spectral types, L and T.
Each spectral type is divided into ten subtypes that received numbers from 0 (the hottest) to 9 (the coldest).
The spectral classification of stars is based on their luminosity class and not on their temperature.