The difference is the width. A string is basically round, whereas a strap is wider. an example would be a bra strap, or a bag strap - if it was a string, it would cut into your shoulder.

A string is thin and sort of 'cylindrical' (hard to explain..) and a strap is flat and is usually used to hold thing up or to hold things together. For example, you hold a backpack up on your back with straps (not strings!). Here is a piece of string: http://tinyurl.com/o3dk8ve . Things similar to "string" are thread, twine, and rope (but rope is usually very thick.) Here is a strap: http://tinyurl.com/ov9rfx7 . I hope that helps!

((Oh! And as for being used in clothing, it gets a little bit confusing I think because depends what it is. The term "spaghetti strap" is a bit of a misnomer. It is used when talking about certain types of women's tops but it looks more like a string...I think this might be an exception though. So, yes; I suppose when talking about clothing you often talk about "straps". String-bikinis or g-strings are also types clothing and they look like they have 'strings' rather than straps.))