Enemies during the early personal-computer wars, Apple Inc. and International Business Machines Corp. said they will cooperate in the mobile era, striking an agreement to create simple-to-use business apps and sell iPhones and iPads to Big Blue's corporate customers.

The deal underscores Apple's push to expand the reach of the iPhone and iPad into the business world—beyond their traditional base among consumers. IBM, meanwhile, is hoping Apple's simplicity and popularity will help stem eight consecutive quarters of year-over-year...