File Formats
Photoshop can work with many different types of file. We can save the image we created in photoshop to JPG or GIF;  photoshop's own file type PSD. Windows BMP, PICT, TIFF, ESP, PDF...etc.

PSD is Photoshop's own file format, saving files in this type will be in large size. Because in this file, every Layer, Mask, Channels, Selections, captions and Copyright with also be saved in this file. Saving files in this format will be  easier to change the image, add more layers, change colors...
BMP is a file format for Windows and OS/2. Saving images in this format will not be compressed, so the size will be large too. But this file format can be viewed in every Windows user,  you don't even need photoshop to view these files.
This is another file format, saving files in this format is compressed, so the file size will be smaller than BMP
PICT is file format used in MAC. If you want to share you images create with photoshop to those who are using MAC but don't have photoshop, you can use this file format.
TIFF - Tagged Image File Format is a sharper and clearer image, can be saved as black & white or in colours. File size is large,normally  used for printing in books, paper...etc.
JPEG can support up to 24 bit (16.7 Million ) colors. Suitable for websites, image can be compress to very small size, and still good looking.
GIF ( Graphical Interchange Format) is a animated images that we commonly see on the internet. But it can support up to 8 Bit, 256 colors only.
PDF ( Portable Document Format)  is also a photoshop's own file format. It can be viewed by those who doesn't have photoshop, but need Acrobat Reader.