How can I get gold versions of cards in Hearthstone?

The first time I saw a premium card I thought that it was like that because it was a Legendary card. Like this:

But I discovered that Legendary cards have a "normal" version:

I thought that maybe it could be explained cause the game is in beta. Like, the normal one was the first version of a Legendary Card, than they became a little more detailed. But searching a little more I found this:

How can I get this kind of card?

The Gamepedia article on Hearthstone card rarity explains gold cards this way:

Cards within the 4 basic rarities described above can have a chance of being obtained with a gold border with custom art animation. Gold cards do not appear frequently, and therefore add to the rarity of a card. Gold cards can also be crafted with Arcane Dust for ten/eight/four/two times the normal cost of the card (in order of rarity). Gold Cards are commonly compared with "foil" cards of other types of Trading Card Games (TCGs).

So basically, expert set gold cards drop randomly when opening packs or can be crafted. Additionally as indicated in this question and answer basic gold cards (which cannot be crafted) are earned by leveling heroes.

Golden Cards in Hearthstone can be obtained from booster packs (more rare than normal cards)and are awarded from placing well (7 or more wins) in a round of Arena Mode. You can also get golden basic cards from advancing high enough with a specific class level, so some are free as leveling rewards. You can craft them with dust, but they cost much more than the normal equivalent. they also disenchant into more dust as well.

I wrote a post for beginner tips for grinding gold and unlocking cards at packs in Hearthstone, since I've been playing since the first day of the Beta.

Check out my gold tips post here:

Hearthstone Tips For Grinding Gold & Earning Packs