Steve Jobs took the stage Wednesday to sell the world on one of his biggest gambles since returning to Apple Inc. nearly 15 years ago: a multimedia tablet-style computer called the iPad. The 9.7-inch touch-screen iPad, which will let users play games, check email, and read books, presents a major challenge to the media, publishing and wireless industries. For Mr. Jobs, it is an attempt to convince consumers they need yet another gadget—one between their mobile phones and laptop computers.