According to http://www.merriam-webster.com/dictionary, a proposal or a proposition is "an act of putting forward or stating something for consideration" and an offer is "a: a presenting of something for acceptance <considering job offers from several firms> <an offer of marriage> b: an undertaking to do an act or give something on condition that the party to whom the proposal/proposition is made do some specified act or make a return promise." Basically, an offer implies something is needed in return, whereas a proposal or proposition is more of an idea or suggestion. suggestion = an idea for something