Utopian socialism was a term coined by Marx and Engels to describe the work of people such as Robert Owen and Charles Fourier. These men shared a similar goal of achieving an egalitarian society as Marx and Engels, but without having any sort of concrete or material basis for achieving said society. The scientific socialism advocated by Marx and Engels argues that there are certain conditions and contradictions existing within capitalist society that will eventually result in its failure the abolishment of class society. Marx and Engels made a teleological argument that communism would be the next socio-economic system that would result from the downfall of capitalism.

So to answer your question, the difference between utopian socialism and marxism is that while marxism is rooted in a understanding of history and economics which argues communism to be the inevitable progression past capitalist society, utopian socialism is a theoretical egalitarian society without any roadmap on how to achieve it.

For further reading, I suggest Engels' Socialism: Utopian and Scientific