Hearthstone currently has 10 playable classes, each of which has seriously unique cards, intricacies, and tons of different ways to play them. Each Class has some specific strengths and weaknesses, and a deck can either be built to accentuate those strengths, or help to cover those weaknesses.
For this list, let's take a look at the Mage Class. Let's get an idea of some of the general ideas/strategies you can use with the Mage Class, and some of the best cards to help flesh out those builds:
10 MINIONLESS
One of the most fun decks in Hearthstone is the minionless Mage deck. That's right, a deck with no minions whatsoever. That's not to say that you won't be using any minions at all; in fact, you're probably going to have some seriously good minions each and every game. But, they're going to be summoned in alternate ways, either with secrets, spells, or by cards that only have an effect in a minionless deck.
9 CONTROL THE BOARD
Possibly the biggest strength of the Mage is their ability to completely control the board with ease. The Mage has a ton of spells that inflict some actually-pretty-serious damage on whatever target they choose (unless the target specifically can't be targeted by spells or Hero Powers), and has a ton of different abilities that freeze enemies, as well. This can make for a supremely frustrating time for an opponent that should have complete board control because they have powerful minions out, but are unable to do anything with them.
8 DRAW/GENERATE CARDS
Because Mages are generally going to be using 2 or 3 cards per turn, it's important to have a lot of cards that either allow you to get some extra Draws out, generate cards on their own, or are Twinspells, meaning they can basically be used twice. If your deck lacks these cards, you're going to find yourself with an empty hand sooner or later, and that's the quickest way to lose a match.
7 MIRROR ENTITY
Though this card fits especially well in a minionless deck, it can be used in any Mage deck. Mirror Entity is a 3-cost Secret that, after your opponent summons a minion, summons a copy of that minion for you.
This usually shouldn't be used right on your third turn, because the value you're going to get out of it usually isn't going to be very high, there. Using this later in the match can get you a really powerful minion for a lot less than that minion really costs.
6 NETHERWIND PORTAL
Another card that fits perfectly into a minionless deck but can be used in any Mage deck is Netherwind Portal. This is another 3-cost Secret that summons a random 4-cost minion for you when an opponent activates a spell. Sometimes you'll get a real stinker of a 4-cost minion here, but more often than not, you're going to get a minion that you can do something with. Plus, it's a 4-cost minion for a 3-cost spell; can't complain about that!
5 APEXIS BLAST
This is a spell that is only worth it in a minionless deck. This spell deals 5-damage to whatever target you desire, and if your deck doesn't have any minions, summons a random 5-cost minion for you. This is a really powerful, bread-and-butter spell for a minionless deck, and every single minionless deck should have it in there.
4 POLYMORPH
Polymorph is one of the basic Mage cards, but it's in the list for a good reason: it can be ludicrously good. This is a 4-cost spell that transforms the target into a 1/1 sheep. You should really pick-and-choose your targets carefully with this; you don't want to use this needlessly on a target that isn't super powerful only to have your opponent play Ysera right after.
But, used on the right targets, this can single-handedly dispatch an opponents plan and send them right into that 'Concede' button.
3 CONJURER'S CALLING
Conjurer's Calling is one of those Twinspell cards, and in the right situation, can quickly fill your board with a plethora of pretty-powerful minions. This is a 4-cost spell that destroys a minion, only to replace it with 2 minions of the same cost. Use this on a high-cost minion (after attacking with it, of course) and you're likely to get some really good value out of it. Heck, if you've got the 8-mana, use it twice on the same turn, turning 1 minion into 3. (Plus giving you any of those sweet Deathrattle effects that they might have.)
2 LEARN DRACONIC
Another bread-and-butter card for the Mage Class (and combined very well with the card above) is Learn Draconic. This is a Sidequest; after playing it, spend 8 Mana on spells to summon a 6/6 Dragon. This is basically like getting a 6/6 Dragon for 1 Mana, albeit not right at first.
1 DON'T WASTE THE BEST STUFF
Don't waste the best stuff. The Mage Class has some really good cards, but used carelessly, they can end up being pretty useless. Playing the Mage Class isn't really like playing any other class, especially when you're playing a minionless version. The cards you play are going to have a variable value based on how you decide to play them, instead of how it is with other classes that are based around playing minions that are pretty straightforward. Play smart, get just a little bit lucky, and the Mage Class is going to take you places.