The term far left refers to ideologies and ideological movements perceived to be further to the left than the centrist left on the left-right spectrum. The extreme left may be used as a synonym or may be used to refer to an "extreme" subset of the far left.
One criticism of such classifications is that the left-right spectrum is too simplistic and there being many very different "left-wing" ideologies outside of the "centrist" left. See also the article on Political spectrum.
The term itself may be seen as negative, as implying extremism or abnormality. It is rarely used by the groups labelled as far left by outsiders.