Thread, string, rope. A thread is a fine strand of fiber used in sewing. A string has a slightly larger diameter. String made of natural fibers would contain two or more three threads twisted together. String is technically rope but of very small diameter used for very light tasks. E.g., Flying a kite would use string and not thread or (a larger diameter) rope. * He had his initials sewn onto his cuffs in gold thread. * He lost his kite when the string broke. * I need some rope to hang this swing on this tree branch. The word "thread" can also be used in the context of following a path. You can talk about a "narrative thread" or "threading clues together". I like that last one since the word clue for a hint comes from the word clew which was a ball of thread or a spool on which thread was wound.

In common use, it's mostly a matter of size, strength, thickness, and use. A "thread" is very thin. You can put it through the eye of a needle. You can use it to sew clothes together. You can easily break it with your fingers. A "string" is thick, maybe 1 mm. It is used for securing things. It is strong. It is too thick to go through the eye of a needle. You don't use it for sewing. You use it to tie packages. A "rope" is even thicker and stronger. It might be one or two cm. thick. You might use it to tie something to the roof of a car. It's used on boats.