You can start any program or app that’s installed on your computer by finding the program’s icon on the Start screen or by searching for it using the Search app, and then clicking that icon. There are other ways to start programs as well. For example, if you see an icon for the program pinned to the taskbar, you can click that. If you see a shortcut icon to the program on the desktop or pinned to the taskbar, you can click (or double-click) that icon to start the program.
Every time you start a program or app, an instance of that program opens in a program window. No rule exists that says you can have only one program open at a time. Some programs even enable you to open multiple copies of the same program. (New Windows 8 apps, however, limit you to running only one copy of that app at a time.) You can have as many programs open simultaneously as you can cram into your available memory (RAM). Most programs allow you to run multiple instances. The more memory your system has, the more stuff you can have open without much slowdown in performance.