Far left is an extremely ill-defined term, used to refer to people or ideas falling into the general category of left wing which the speaker considers to be far into that spectrum.
The term must be understood as relative to either the speaker or the context in which they are speaking. So a US conservative may consider any member of a Communist party to be part of the 'far left', while in a communist state there may be 'moderate' communists and 'far left' communists.
In general, it is used to describe persons or groups who hold radical egalitarian views and who seek to achieve radical social and political change by taking over or overthrowing the existing order in society. Depending on a group's militancy and the given political circumstances, revolutionary change may be achieved through democratic or undemocratic, violent or nonviolent means.
Groups that advocate some form of revolution, but without being egalitarian, are generally not part of the far left.
Typically, far-left groups are communist or anarchist, and they strongly oppose capitalist governments and institutions. Sometimes the term "far left" is used pejoratively by those on the right wing to describe any view they perceive as hostile to capitalism.
See also