Maybe a simple question

A primer is here:
http://www.wfu.edu/~matthews/misc/graphics/formats/formats.html

The "strings" are the type of file it is. Referred to as file types, they are also known as file extensions.

JPEG (jpg) is a compressed file format, which has smaller diles but sme quality loss.
It is usual to send emaied photos as a .jpg file as they could be too large otherwise.

.TIFF is a non-compressed format, as is .BMP

Note that you cannot change file types by renaming the extension, it has to be done with image conversion software

I use a free program named IrfanView to convert images from one filetype to another. You can get it at http://www.irfanview.com/ and it is a great image manipulator. There are other programs that will do similar things, but none as easily. It is as simple as opening the image in IrfanView, and using "Save As"' to save it as a different filetype. Just select the new filetype from the drop-down menu.