Thru is a short spelling of through. It sounds exactly the same and means exactly the same; only the letters are different. It is American, and it belongs to signs, forms and quick notes rather than to real writing: 'open Mon thru Fri', 'drive-thru', 'thru traffic'. Two places you will really meet it. First, 'drive-thru' — the window at a fast-food restaurant where you order from your car; that spelling is now the normal one, even on the company's own signs. Second, American dates and ranges, where 'Monday thru Friday' means Monday to Friday and includes both ends. Everywhere else, write through. If you use thru in an essay or a work email, an examiner or a British reader will very likely count it as a mistake. Interestingly, this is not a new lazy invention — 'thru' was one of several spellings floating around in Middle English, centuries ago; the modern American use grew up around 1839 as a simpler, more phonetic way to write the word.