- For the ability, see Tradeable.
A trade is the loss of one or more cards in exchange for the elimination of one or more enemy cards. Most often, trading refers to minions dying in combat, but the term may be extended to refer to spells and weapons as well. Its several variations are used to identify the tactical advantage gained or lost in the exchanges.
Discussion[]
The simplest type of trade is an "even trade", where two single minions kill each other in combat. For example, two opposing 3/2 Bloodfen Raptors would both die if one attacked another; neither player gains a clear advantage from such a combat. Trades can occur over the course of multiple attacks, for instance, two Sen'jin Shieldmastas could attack each other in 2 separate combats and result in an even trade.
Minions may "trade up" or "trade down" based on the mana cost of the minions involved. For example, a 1 mana 
Injured Kvaldir can kill a 2 mana 
Bloodfen Raptor, creating an advantage for its owner, who could then cast their own 2-drop without it being at risk of death from the Raptor. Trading up does not create card advantage since 1 card is still lost by each player, but it can create a tempo advantage by ensuring that the player's limited mana is used to greater effect than the opponent's.
If a minion is killed by another without killing it in return, it has failed to trade with its killer. Another minion or spell will likely be needed to finish off the enemy, resulting in an overall "2-for-1" trade, generating card advantage for the player losing only one card. For example, a Chillwind Yeti killing an Ogre Magi is left at 1 health, and may then trade with a Thrallmar Farseer. Higher numbers may be used the more cards are involved in the interaction: A Blood Knight may kill a Frostwolf Grunt, be killed by a Dalaran Mage, which is used to hit a Silvermoon Guardian, which finally trades into an Emperor Cobra - a 3-for-2 exchange in favor of the Blood Knight and Silvermoon Guardian. In the same vein, a series of interrelated combats may result in an approximately even trade even if each individual combat was lopsided in one direction or another.
This concept can be easily applied to spells and weapons. For example, a Fireball used to kill a Chillwind Yeti is an even trade, since two 4 mana cards eliminate each other from the game. An Earth Shock used to kill a Twilight Drake is a significant trade up, since a 1 mana spell was used to kill a 4 mana minion. Area of effect spells such as Blizzard often create many-for-1 trades by killing multiple minions with a single spell. Weapons can allow several-for-one trades since they can attack more than once without being destroyed.
Minion special abilities can be taken into account when identifying trades. For instance, a 
Bomb Lobber that kills a Salty Dog with its Battlecry before trading with a Blood Knight has given its owner an excellent 2-for-1 trade. Less direct abilities can be more difficult to judge; for instance a Gnomish Inventor trading with a Magma Rager has traded down slightly from the tempo perspective of cards on the board (it cost 4 mana to get Gnomish Inventor in play compared to 3 for the Magma Rager), but has created a 2-for-1 (or arguably a 1-for-0) trade in terms of card advantage, because the Gnomish Inventor essentially drew a replacement for itself via its Battlecry.
"Favorable" and "unfavorable" trading can refer to generating tempo, card advantage, or both, and is an important skill for selecting attacks and casting minions and spells for their fullest potential.
For a deeper discussion of strategies, see Card advantage and Tempo.