What is Overload in Hearthstone?
Hearthstone has many card effects that are common enough that they get their own keyword - a word or two that immediately tells you how the card works. Let's take a look at the Overload keyword.
You have X less mana next turn.
Overload is a mechanic found exclusively on Shaman cards. Playing a card with Overload will lock X of your Mana Crystals on your next turn. On paper, this seems like a strictly negative effect, although it does have some upside. If you perhaps weren't planning on spending all of your Mana next turn anyway, cards with Overload will allow you to get a boost in tempo with no drawback. There are also certain cards that benefit off of Overload. Do be warned though as Overload will require proper planning to be used effectively. If used willy-nilly, you may find yourself starved for mana. As such, Overload is a mechanic that rewards proper planning ahead and punishes recklessness.
Cards like Likkim will count Overloaded Mana Crystals that are currently active this turn, as well as any that are "banked" for your next turn. Lava Shock and Eternal Sentinel can also be used to unlock your Overloaded Mana Crystals.
Just like other pseudo-Battlecry keywords, Overload will not trigger if a minion with it is cheated out of your hand or deck without playing it. Unlike those other keywords however, spells with Overload that are cast with cards like Yogg-Saron, Hope's End will still Overload you for the specified amount.
Keywords that Work Great with Overload
Minions with Overload will work well with practically anything, and because they're cheaper than they otherwise would be, granting them these through card effects are easier. Cards that Overload you will also work with cards that benefit off of Overload triggers such as Unbound Elemental or Vessina.
As spells with Overload are often cheap with strong effects, Spell Damage can increase its reach even higher, granting you an even more mana-efficient board clear.
Playing Against Overload in Hearthstone
Overload minions are large for their Cost, but can still be defeated with hard removal, which will end up leaving them Overloaded with no benefit.
Paying attention to how much they Overload can help you plan ahead better. For example, if your opponent plays an Eye of the Storm, then you can take note of the fact that your opponent only has 7 mana left on their next turn, meaning they won't be able to play any cards that cost 8 or higher.
Example Overload Hearthstone Cards
Flamewreathed Faceless is a simple, and fairly notorious example of Overload in the game, providing a massive 7/7 stats for only 4 mana whilst Overload the player for 2. This card is where the famous "4 mana 7/7" meme originates from.
Tunnel Trogg is a strong aggressive card that gains +1 for every Mana Crystal you Overload while you control it. Surging Tempest is a nerfed version of the card printed in Descent of Dragons.
Elemental Destruction is a huge board clear that will deal 4-5 damage, often clearing many board states, to everything for only 3 mana. It however boasts a hefty Overload of 5, which is the highest Overload of any card printed in the game (unless one counts the limited time Arena-exclusive, Crackling Doom).
Lava Shock is a spell that will deal 2 damage and unlock all of your Overloaded Mana Crystals, making it effective if you find yourself Overloaded in mass. It is one of only 2 cards in the game with this effect (the other being Eternal Sentinel).