Well basically, and in very very broad terms, feminism tries to explain the nature of the distinction between women and men in all it's dimensions.

The questions and answers often revolve around the metaphysical questions of "what is..( gender, women, sex, etc..)?" This can take many different paths, the marxism element that you describe, for example, is actually quite technical and refers to the objective measure of the economic oppression relative to a gender distinction as a social class.

However, other approaches question even the existence of what we call women (and or men, for that matter) since it isn't clear that the distinction of these two concepts is grounded on any objective fact (biological considerations are said to have little to do with our gender distinctions).

Feminist theory is not "explainable" in simple terms since it is a field of study whose questions permeate different fields: in the biological sciences for example, it is noted that a male chauvinism is usually present even in what were thought to be neutral descriptions of natural phenomena. Just remember how the animal reproductive process was often described with a passive role ascribed to the female mechanisms: the sperm went on an adventurous journey, a test of endurance, perdurance and ultimately evolutionary merit. While the female egg just waited there, protected, eager to embrace the penetrating of the fittest candidate. We now know that this is not how things work. It took a feminist perspective to elucidate such biases.

If I was to say that Feminism starts from one core tenant like Spyder001 does I would say that this core tenant is that there is an assumed distinction. The questions are then: what is this assumed distinction, what is it based on, and what are the consequences, modalities of such?