What is this "left", said again and again in the assenbly election held in WB and Kerala?

"Left" is nothing but a group of political parties who believe in communism. These are Communist Party of India(CPI), Communist Party of India(Marxist) (CPI(M)), Forward Bloc, etc. There ideology is same, i.e. Communism. The Left parties have ruled West Bengal for last 34 years, Kerala for last 5 years, and still ruling Tripura. As Left have no other states that they are ruling, so their defeat is a matter of national interest.