Art decos are small (usually postcard size or a little bit smaller or bigger) decorative booklets consisting of 4-12 pages and contributed to by several people. They are a hobby that keeps the mailman working as the booklets are swapped via snail mail. Many people have personal swappers with whom they exchange swaps or they join organized swaps for example via yahoo groups.
Art decos have developed from fb's (friendship booklets with only basic information in them) & deco fb's aka decos (friendship booklets decorated with stickers & pictures) in the early 2000's. They were highly influenced by mixed media & paper arts and scrap booking. Art decos are a more artistic and refined version of decos.
Sometimes the categories are quite fluid but the biggest difference between decos & art decos is that in art decos you never cover your page with contact plastic. Also, art deco makers & swappers prefer more creative ways of binding the booklets than just stapling them like most of the basic decos are made.
If you get inspired and feel like trying making & swapping art decos there are lots of groups to join.
My favourites are: My own group Discover_decos of course: https://groups.yahoo.com/neo/groups/Discover_Decos/info
Addicted_to_Vintage which is dedicated to vintage themes: https://groups.yahoo.com/neo/groups/Addicted_to_Vintage/info
EuropeArtDecos for European swappers: https://groups.yahoo.com/neo/groups/EuropeArtDecos/info
Euro Deco List (another European group): https://groups.yahoo.com/neo/groups/EuroDecoList/info