Which is which and what do they mean? I know Freund is friend and Freundin means girlfriend. Am I correct?
Any time you see -in on the end of a noun (like an occupation, etc), it's the female version. Freund is male friend (and boyfriend), Freundin female friend (and girlfriend). Freunden is multiple male (or mixed male and female) friends, Freundinnen is multiple just female friends. See also: Student/Studentin, Schüler/Schülerin, etc. Think of it like actor/actress in English.