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.