@candle
compose key: extra modifier you hold down to make new characters out of two keystrokes (e.g. ae>ä, sz>ß, e'>é). I think it's only available on Linux (you can map it on Menu, for instance)
dead keys: ISO keyboards have accent keys (^° and ´`), that you push first, then a letter and if the accent can go over the letter, it'll create the symbol. They're called that way because just pushing them doesn't input anything (you can press space to get just the accent)