Answer:
You can save from Photoshop in many formats like JPEG, PNG, TIFF, PDF. Difference is because TIFF can save layers, PDF can save vector data while JPEG and PNG can save only flat image or only pixels. There are and many other formats to save or export image from Photoshop but usually people work and save all their images in PSD format because this format supports all Photoshop capabilities and can keep most of them as editable. Off course when you need image for Web you will save it usually as JPEG to post it but keep original always and my advise is to keep it in PSD format on your computer.