The Chrome OS, which runs on top of the Linux kernel, is a direct challenge to Microsoft's ( MSFT) dominance in the PC market and will also intensify competition between Google and Apple. Chrome is expected to launch in the second half of the year and is an extension of Google's Chrome Web browser. It will initially be targeted at netbooks -- and tablets. The site says that Chrome will explore the possibility of creating multiple browsers on tablet screens. On another page, Google explains that, for tablet computers with 5-inch to 10-inch screens, the user interface would be adjusted to handle "larger touch targets" such as icons. The mock-ups feature different-sized touchscreen keyboards as well as tabs alongside the screen. The side tabs offer links to programs like Gmail, although the site says that Google is exploring two other forms of interface, "classic" and "compact." The classic style uses a single maximized window with a URL tab at the top of the page, whereas the compact version removes the address bar.