To print your files and maintain their original quality it is advisable to save them to a .tiff format.This format is larger than the commonly used .jpg file format but it will not compress the contents as much and therefore preserves it as intended by the artist. The standard resolution fro a print file is 300dpi.
If you are intending to use your work on a website or to email files to selected clients or galleries then it is advisable to save a .jpg copy, this should have a resolution of 72dpi to ensure it is small enough for email servers to send and receive. You can also email and post images on the web in .gif or .png format but this is only advisable for certain image types as these can distort some colours and edges in photographic work.
Other common file formats are .psd, .pdf and .ai or .eps, these refer to the program that created the original file (Photoshop, Acrobat or Illustrator). Some professional printers will be able to work with these files but you will need to check that your intended recipient has the software before you send it. It is also worth noting that the older versions of these programs cannot open files created in the newer versions. If the person you are sending you file to has an earlier version of the application, you will need to save a copy of your file in a legacy format (compatible with earlier versions) so they can open it. Adobe Reader can also open .pdf documents and is available for free download, however you will not have access to all the tools available in Adobe Acrobat.