Marxism refers to the theories of Marx (and by extension, Engels too). But, Marx and Engels were philosophers and theorists for literal decades, and they said a LOT of stuff, some of which is directly contradictive.

Anyone who brings up the term 'Marxist' in a modern political discussion should automatically be ignored. Whatever they are about to say is going to be some right-wing, tin-foil-hat-conspiracy. 

Modern progressivism has evolved beyond 'Marxism' for literally 2 centuries. There aren't American Marxists in any position of influence. Bernie is not a Marxist, socialist, or communist, and anyone who claims he (or those like him) are, has an ax to grind with progressive ideology. Their argument will not be in good faith, regardless of whatever point they are trying to make.

Karl Marx is one of the most influential economic theorists of all time, but his work was literally centuries ago, and no modern politician is reading Das Kapital for pointers.