The term far right refers to ideologies and ideological movements perceived to be further to the right than the centrist right on the left-right spectrum. The extreme right may be used as a synonym or may be used to refer to an "extreme" subset of the far right.
One criticism of such classifications is that the left-right spectrum is too simplistic and there being many very different "right-wing" ideologies outside of the "centrist" right. 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 right by outsiders.
In practice, the term (and derivations such as "extreme right") seems to be mainly used by critics, who often seem to use it very narrowly as a synonym for "supremacism" and in particular for "White supremacism".