At the word's simplest meaning, a comrade is a companion either through friendship or related interest/situation. Today it is somewhat associated with communism due to its use by socialists and communists as a method of addressing someone that doesn't imply a status-level. Similar to companion or fellow. The communist use of the word is said by many dictionaries to have originated during the French Revolution and was first recorded in English with this usage in 1884.1
The word comes from the French camarade, which in Old French translated to roommate. This came from the Old Spanish word camarada, meaning much the same thing as the French version but especially related to companions in a barracks. Camarada was derived from camara for room, which originally was the Late Latin word for chamber. Comrade's first recorded entry into the English language is in the 1500s.2 The word comrade does not originate from the Russian language, as many people likely believe (Albert Herring informs me tovarich is the Russian word generally translated as comrade, in case anyone would like to know).
During the United States' Red Scare period, calling someone "comrade" within the States was usually an indirect method of accusing that person of being a communist. During this time people accused of being communists generally weren't anything more than a bit further to the left on the political spectrum than the accuser (or the accused was disliked by someone with power or powerful friends). Today there's no threat of being imprisoned or blacklisted for being a communist (or being accused of one). I'm not sure if this has more to do with society coming to its senses over the years or just the current absence of any supposedly communist superpower. In any case, addressing someone as comrade today probably won't get anymore than a funny look, if even that.
1Dictionary.com, The American Heritage Dictionary of the English Language: Fourth Edition (http://www.bartleby.com/61/99/C0539900.html)
2sources listed in footnote 1 put the word's first official appearance sometime in the 1500s
http://www.geocities.com/etymoline/c3etym.htm states 1591
yourDictionary.com states the entry of the word into English as 1544
None of these state what work the word appears in. Presumably, it could just be a dictionary from those years, though this is merely speculation. My searches for more information on this haven't turned up anything more.