A heap is the total of all collected cards of a player. The heap is actually only used when in quest mode, otherwise all cards are available and a "quick"-player can use all cards which are available. To build a deck for a non quest player - all cards can be used for building a deck.
For quest players the cards to build a deck from are restricted to the onces in the players heap.
A heap for quest mode (at the time of writing) starts of with the original portal starter contents, which means a new player has exactly 70 cards and one booster with 15 random cards and a land.
Which also meeans, that a starting player has a maximum of 1 rare cards.
In order to get more cards, the player must earn money. This can be done by
doing quests
just playing (and winning) against computer opponents (usually about 5$)
selling cards in the shop
for the money the player earns, the player can buy boosters in the shop.
Or
The player may by successfully solving a quest get directly boosters and/or cards as a reward.
Anyway, once the player starts to play in quest mode, he will allways have a heap. The heap can be viewed via the following screen:
Heap
The heap itself is not editable, and can only be changed by gaining new cards or selling cards in the shop.
Note: This view holds ALL cards as single entries, thus 5 plains will be visible as - well - five plains and not plains *5 :-)!
The button on top of the screen (also available in the player information panel) gives you access to the "collection booklet" (see next chapter).