Digital kits, also know as digital embellishments, elements and papers, are what is used for digital scrapping. Much like its paper counterpart, they are what you will use to make your scrap pages your own.
You can buy kits from several stores/blogs or even find free kits by searching online. Whatever your taste or style, you will be able to find what you are looking for.
Most digital kits, unless labeled "Tagger sized" are full 12x12 sized papers with the resolution of 300dpi. The embellishment/elements will be of various sizes, though they can be sized to your project needs.
A popular "freebie" kit search engine is http://digifree.craftcrave.com/
You can also find free kits here at Shabby Princess
and of course you can download my freebies and purchase my kits I have on my blog.