Graphic files come in many different formats, and each format can be used in different types of software and have different sizes etc. In the main you don't need to worry about them, but here is a summary of what the main differences are and what you would expect from a digital/printable crafting download
files that end in .png are the highest quality. They are also generally take up a lot of space and take longer to download, so they tend to be used for smaller items wherever possible.
Also png files are also the only format that can have transparency, so they are always used for digital scrapbooking elements and quickpages that have holes in for you to put your photos behind.
files that end in .jpg can be compressed - that means that the file doesn't get as big as it would if it was a png. This is the most common format for digital papers.
tiff / bmp/ gif/ wmf
Some older software cannot use jpg or png files. So they use these other formats. Some are low in quality, some produce absolutely enormous file sizes. These are rarely used for downloadable printable/ digital scrapbooking files.
These files are quick animations that end in .gif and are typically used on internet sites.
.pdf files are documents that open in the free Acrobat Reader software from Adobe. Electronic Magazines and e-books are in this format. Many designers (myself included) produce printable products as .pdf files to make it easy for the customer to open and print them