Hello !! 'things' is a Countable noun and is used in a variety of situations both formal and informal.... "i've got a few things to do today", "he said all the wrong things in the interview", "there are several things that I left at home". 'stuff' is an Uncountable noun that we use in very informal situations only with friends and family... "loads of stuff", "i've got to do some stuff today", "don't forget your school stuff!". VERY INFORMAL. I would avoid using stuff where possible, and certainly not in written English. English is a very rich language and there are plenty more intelligent words to use as a replacement ! I hope this has been helpful :-)