The deck builder allows the user to construct and customize decks. You can have multiple decks at once.
A deck must contain at least 40 scrolls and you can only have three of the same scroll, you can otherwise construct them however you like. Any combination of creatures, structures, spells and enchantments of any resource type can be used together. There doesn't seem to be an upper limit to the number of scrolls you can have in a deck, but it reaches at least 180 scrolls.
You can arrange your deck however you please in the lower half of the Deck Builder screen. You can also click the button in the bottom right corner of the Deck Builder to auto-arrange your deck (scrolls are grouped together and ordered by scroll name). The arrangement of the scrolls is saved when you save your deck.
All the scrolls you own, except those in your current deck, are in your library. You can filter the scrolls shown in your library by resource type. You can also sort them by Name, Cost, Type and Resource. To move scrolls to and from the library, simply drag and drop the scroll you want either into the pages of the library, or anywhere you like in the deck builder.