What is the difference between polar and charged amino acids?
All amino acids with side chains that are charged at physiological pH are, by definition, also amino acids with polar side chains (e.g., lysine or glutamic acid). The converse is not true; in other words not all amino acids with polar side chains are necessarily amino acids with side chains that are charged at physiological pH (e.g., threonine or asparagine). The pKa of the nitrogen in histidine's imidazole ring is quite close to physiological pH, and in some situations would be charged and in other environments would be uncharged. In either case it would be considered polar (or largely hydrophilic in nature).