(A) Political Spectrum: The terms 'left' and 'right' derive from where people used to sit in the National Assembly in the French Revolution. Right means reactionary, wanting to go back to an old system, or preserving the current system, including social hierarchies or inequalities in wealth. Left means wanting to change, reform, or abolish, which includes creating equality of wealth. Those on the far left want to abolish private property and create a society in which everything is owned (and controlled and distributed) collectively ' in practice this means nationalised ownership by the state, and control by the ruling communist or socialist party.
(B) Parties on the far left: The Revolutionary Internationalist Group in the 1950s evolved into the Militant Tendency in the 1970s, and started a process of infiltrating the Labour Party as a way of controlling it. These Militants were gradually expelled from the Labour Party in the late 1980s and early 1990s, and formed the 'Socialist Party' in about 1992.