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Each of these -isms are an ideology, or 'coherent set of beliefs'. So they entail a core set of compatible beliefs around which other ideas are formed and from which policies are made in mind.
Conservatism: Fundamentally, a resistance to change. A belief that there is a 'natural' order in the world and that if any changes are made, they should be done in small, incremental steps. A dislike and fear of big changes and revolutions. A strong belief the need for historical identity - especially national identity. A stress on the importance of the needs and wants of the family and individual over society.
Socialism: Socialism and Communism are sometimes used interchangeably. I will define Communism separately, and define 'Socialism' to mean 'Social-Democracy'.
Social-Democracy: The belief that society can and should be shaped for the welfare of all stakeholders in society. A strong emphasis on the importance on health and education. Whilst preserving Capitalism, the belief that Capitalism must be strongly regulated and kept in check in order to work properly. Capitalism without rules and regulation results in barbarity. Modern nation states like France, Germany and Sweden like to think of themselves as modern 'Social-Democratic' states.
Marxism: An ideology based on the writings of Marx. And there is more than one interpretation possible, as people have been discussing and disputing Marx for over a century. Fundamentally, the belief that the history of the world is a history of class conflict: between slave owner and slave, lord and serf, Capitalist and worker, and that man is capable of shaping his own destiny in the world through struggle, ultimately improving his lot.
Communism: The ideal outcome of historical struggle as believed by a Marxist. Classes have been eradicated, and man is in full control of his potential, having seized the means of production, and is now creating wealth, learning knowledge, and creating technology to serve mankind as a whole, rather than a small percent of the ruling class at the top.
Liberalism: It depends whether you mean the political or economic system of thought. Politically, it stresses the freedoms and rights of the individual over society. The fundamental belief that people can do as they please so long as they harm no one else. Economically, it means laissez-faire Capitalism. The belief that markets are best left to their own, and any interference with the market ultimately is more harmful than good.
Hope this helps. :)