済ませる:すませる： to finish homework, chores. "No dessert until you finish your spinach!"
完成:かんせい： to complete - usually said of a painting, construction of a building, or other physically creative project.
完了:かんりょう： to finish work on a project - an annual report, a re-formatting of the database, etc.
終了 : しゅうりょう： something (a contract, a subscription, a statue of limitations) expires or terminates.
結末：けつまつ： the conclusion of something - the last chapter of a book or the last scene of a movie.
終わり ： おわり： "the end" - usually written at the end of old movies. Also said out loud by someone who wants to declare victory and change the subject.