Other File Formats You've Probably Run Across:
What it stands for: Photoshop Document
What it's used for: Default Photoshop file. Supports layers as well as image effects, grid overlays, and all sorts of Photoshop specific abilities. These files generally tend to be very large as they are uncompressed. You'll generally use this to save works in progress, and when finished, you'll save as a JPEG, GIF, or PNG file. Several programs such as GIMP, Open Canvas, and Corel Painter can open PSD files in some capacity.
.BMP (or its counterparts: .RLE, .DIB)
What it tands for: Bitmap
What it's used for: You've probably run across when you played around with Microsoft Paint, as Paint saves all the images as .bmp files. Often used as a texture file. Files tend to be rather large, because the image stays fairly uncompressed. Also, a lot of scanners will scan images in as .bmp files.
.TGA (or its counterparts: .VDA, .ICB, .VST)
What it stands for: Targa
What it's used for: Texturizing. It works in the same way that a .PNG file does. The popular online game Second Life uses this file type to texturize clothing for custom characters. Programs such as Maya, 3D Studio Max, and Poser also utilize this file type as well.
.TIFF (or its counterpart: .TIF)
What it stands For: Tagged Image File Format.
What it's used For: Scanners often scan in .TIFF format, as it is simply a high-quality Bitmap file. Some programs (such as Photoshop) can take advantage of the fact that TIFF files actually do support layers.