They're actually the same thing; transfeminism has become the popular term for postfeminism in America, because conservative groups (like "Feminists" for Life) in the early 90's began misusing the term postfeminist to discribe conservative women who thought feminism was pointless or irrelevant. So to avoid being misassociated with those groups, American feminists began using transfeminism and fourth-wave feminism while theorists in Europe like Helen Cixous & Julia Kristeva, who were inspired by popular French postmodernist philosophers, kept using postfeminism.
I guess the best way to describe postfeminism is a blend of feminism and queer theory. The earliest feminists focused primarily on the oppression of womyn. Next came along second-wave and third-wave feminists who questioned that idea, as they saw feminism as both a womyn's and men's issue. Postfeminism is basically a challenge to both of those notions, and a criticism of using simple binary systems to classify an oppression as broad as sexism which clearly transcends woman/man. Judith Butler and Julia Kristeva are probably the most famous postfeminists, if that helps you grasp postfeminist theory any better.
