This is an excellent question. If you did not know, feminist theory is a massive body of literature that spans over a hundred years. Moreover, a lot of the theories that are involved are difficult to read. In light of this, I will give you a brief synopsis of some of the major theories.
First, a lot of feminists have worked on the area of epistemology. They explore the production of knowledge from a woman's point of view instead of a man's perspective. One of the aims of this is to liberate a woman's perspective from the discourse of male ideologies.
Second, feminist theorists have also challenged the objectification of women's bodies. This is particularly an acute issue, because there are so many body image issues in our society when it comes to women. In addition, there is gross inequality in view of the fact that often times men are associated with their minds.
Third, feminist theorists have challenged to role of language, that is to say, that our language itself is so masculine in its nature that it does not accurately relate women's experiences.
To get deeper in any of these topics, it is good to read an introduction. Here is one of them: What is Feminism? An Introduction to Feminist Theory, by Chris Beasley.