English has a lot of concepts for which there are two words: one descended from the Germanic family, and one descended from the Romance family of languages. This is a perfect example. "Freedom" is related to the ancestor of the modern German word "Freiheit," while "liberty" is related to the ancestor of the modern French word "liberté." We English-speakers just make our own version of both words and use them both in English. :) In usage, I would say that "liberty" is more commonly used in political/official language, while "freedom" is more commonly used in historical language when referring to the emancipation of the slaves in the US. They are both emotionally-charged words, but I would personally say that "liberty" is a little "fancier" and therefore slightly more emotionally distanced. If a soap-box speaker in a park really wanted to excite the common people, s/he would probably prefer to shout about "freedom" rather than "liberty."