How to understand the rarity of a card

My experience, paired with documentation gives this simple explanation.

• Growth: Cheap(ish) units that have a lot of effects or have spells that help them grow or summon other units.
• Energy: These scrolls hit hard and are intended to do a lot of damage, quickly.
• Order: Order scrolls tend to be rather weak as single-units but can gain strength when other order units are played in close proximity to them. Many of these units give a lot of bonuses to their fellow units when they are near by.

In my opinion, the Order type requires the most game-play strategy while the other two types can do well from a basic deck-level strategy and a bit of luck. It's been argued that this makes the game rather unbalanced, against Order.

Yes, you get gold just by playing and all cards are obtainable just by playing.

In single-player:

• You get gold by playing a quick match (both winning and losing).
• You get no gold for the tutorial match.
• You get gold once for each trial you win. Repeated trials give nothing.

In multiplayer you also get gold both for winning and losing a match. For details you can check the wiki.

Shards can only be bought with real money and can be used to:

• Unlock the other starter decks.
• Buy the weekly face-up offers. These are six random scrolls (2 common, 2 uncommon and 2 rare) that refresh weekly just for you.

Shards can not buy anything that can't be bought with gold, and to buy random scrolls and scroll packs (which is the primary way to get new scrolls) you can only use gold.