There are 4 tab of collections. Each one allows you to unlock or purchase cards after fulfilling certain requirements.
The first tab is for completing campaign stages and unlocking the 'shards' that appear as random drops for replaying.missions.Shards look similar regular cards, but are only colored in red and black cracks. If you collect enough of them, you can go to the collections area a set of these shards for for a regular card.
The second tab under collections is for shards collected in the dungeons.
The third tab is for cards earned by 'honor' in the arena. You have to win at least 15 matches to get to 'bronze' before you can get any of these. This is an alternative purchase page for the honor tab in the shop.
The fourth tab is for cards that you can get by completing 'achievements'. They each refer to how many times you've won for each class. You unlock the first after 5, then 20, 100, 200 and finally 500 with increasing rarity and they directly compare to the achievements of each class for wins in arena.