loop is a very powerful, complex iteration macro which can do nearly anything. Literally. It has its own language built into it. loop is one of the few things in Lisp more complex than format.
loop has an idiosyncratic syntax that is very un-lisp-like. It is also so complex that few people understand it, and it is not recommended for use. We will not discuss loop except to mention that its very simplest form: (loop expressions ... ) makes a very nice infinite loop.