In most programs, opening files is simple and that holds true in Photoshop, too. But Photoshop gives you a few more options than you'll find in some other programs because it can open files saved in its own format (PSD files) as well as a slew of other formats both common and obscure (a feature that makes Photoshop amazingly versatile at working with a wide range of images). Even better, if the program doesn't recognize a particular format, there's a good chance someone has developed a plug-in (companion program) that can. (See Chapter 19 for more than you ever wanted to know about plug-ins.)
Photoshop knows how to open Adobe Illustrator, Camera Raw (Working with Raw Files), JPEG, GIF, PNG, TIFF, EPS, and PDF files (Working with PDFs), along with Collada DAE, Google Earth 4 KMZ, Scitex CT, Targa, and several other file types most folks have never heard of.
You can open files in Photoshop in several ways, including:
Double-clicking the document's icon, no matter where it's stored on your computer.