Boy, is that ever an
understatement.
Wyclef pretty much had it when he said "There are at least as many versions of
Socialism as there are of
Capitalism."
Communist Manifesto era
terminology, where
Marxism,
Communism, and
Socialism were one and the same, is pretty much irrelevant now, what with the advent of
Marxism-Leninism, the more recent need for the left to dissassociate itself from
Stalinism, and so on. But, at least in
Europe (this does not apply to the
US where the word still enjoys
cussword status),
definitions are beginning to consolidate and
socialism nowadays means little more than
state ownership of just the
major industries, and
Socialist parties subscribe to this
doctrine and occupy a
halfway libertarian stance on (most)
social issues. Basically the equivalent of the far left of the
Democratic Party in the US.
Marxism has become synonymous with relatively moderate Marxism-Leninism (oxymoronic but true, in most countries they're not legally allowed to advocate revolution), which has also become more or less synonymous with Communism, because that's the stance most European Communist parties hold. (Note -- there are generally rifts in the Communist parties between the pure Marxists, the Marxist-Leninists, the in-betweens, the whatevers, etc., etc.) And the two segments of the left pretty much hate each other as much as (or more than!) than they hate the right: Communists/Marxists think Socialists have sold out, and Socialists think Communists give the left a bad/radical image.
So no, Socialists are not necessarily Marxist - infact, these days they almost never are.