What is the difference between a Cicada & a Locust insect? why these come around only after so many yrs?

A locust is a kind of grasshopper or a Orthodopteran. A cicada is a true bug or a Hemipteran.A cicada stays under ground for several years in its grub, or larval stage, eating things like roots, I think.You can correctly say a locust is a grasshopper and that a cicada is a bug.Hope this helps.Source: Zoology course.