Sometimes it's not clear to new players what exactly a spell will do. Do you remember whether the Boar Rider will give you credit for the gems they take off the board, or if the Lance Knight does? What does "double damage if the target is Fey" mean? Here's the cheat sheet.
Adjacent: The troop(s) right next to the spell's target. Troops with a gap between them are not adjacent.
Ally: One of your own troops, including this one unless explicitly stated. (E.g., Brian the Lucky only affects "other allies", not himself.) The phrase "an ally" means you choose, as opposed to "random ally", "all allies", etc.
Beast: Animals. Any troop with a paw symbol on their card.
Boost: Often a spell is boosted by making a change to the board, such as removing gems. There is a boost ratio with each spell. For instance, if a spell does 4 damage boosted by removing green gems, and has a 2:1 boost ratio, it does 1 more point of damage for every 2 green gems removed.
Cleanse: Undo negative status effects like entanglement, poison, etc.
Column: An entire vertical column of gems. Almost always used with "destroy" or "remove", and almost always caster's choice of column.
Create gems: New gems of one or more specific colors are created. Gems that were already those colors are not changed. Spells that create gems may result in 4- or 5-of-a-kind and provide an extra turn. Or they may not; use with caution.
Daemon: Any troop with a daemon symbol on their card.
Damage: Does damage to armor first, then life, just like skulls do--unless it specifically says damage to a skill (like true life). If no target is specified, it's the first enemy troop.
Destroy armor: Permanently reduces a troop's armor to 0. (Armor can be regained via other spells.)
Destroy gem: When a gem is destroyed, you earn mana for it. If it's a skull, it does damage to the enemy. The gem destroyed is your choice unless otherwise stated (random, all of one type).
Devour [gems]: Same as "remove"--no mana is gained nor damage done. Usually something is gained in exchange.
Devour [troop]: Destroy a troop and add all of their skills to this troop.
Dragon: Dragons. Any troop with a dragon symbol on their card.
Drain mana: Drains mana but does not steal it.
Elemental: Any troop with an elemental symbol on their card.
Enemy: An enemy troop. The phrase "an enemy" means you pick the target. Other options may be "random enemy", "all enemies", "first two enemies", etc.
Entangle: An entangled enemy can't do damage by matching skulls, unless they match extra. (Effectively, their attack is temporarily reduced to 0.) Entanglement will wear off eventually, with an increasing chance each turn.
Explode: Destroys a gem and the 3x3 area around it, but only earns 70% mana. The gem exploded is your choice unless otherwise stated (random, all of one type).
Fey: Fairies. Any troop with a butterfly symbol on their card.
Full: A skill's maximum value. This isn't just the regular value; it's as high as the skill ever was for this troop in this battle.
Gain an extra turn: Exactly what it says on the tin.
Gain [skill]: Adds to the specified skill.
Heal: Same as "restore life".
Hunter's mark: This enemy takes double damage from skulls. Will wear off eventually, with an increasing chance each turn.
Poison: A poisoned troop has a 50% chance to lose 1 point of life on each new turn. (It is only counted when control first passes to you, not during extra turns.) Life will never drop below 1 point from poison. Poison does not wear off, but it can be cleansed.
Random: You don't choose this target (troop or gem).
Remove: When a gem is removed but not destroyed, you don't earn any mana for it. Same with skulls: they don't do damage. When removing a row or column, the row/column is your choice unless otherwise stated.
Restore [skill]: Restores a skill, up to its maximum value. Skills boosted past their maximum will always raise it, so a troop that has gained extra life has a new, higher maximum.
Row: An entire horizontal row of gems. Almost always used with "destroy" or "remove", and almost always caster's choice of row.
Silence: A silenced enemy can't collect mana or use their special skill. Their mana is also drained when silenced, so it doesn't come back when the silence is lifted. Silence will wear off eventually, with an increasing chance each turn.
Skill: One of the four skills: attack, armor, life, or magic. Usually used in the context of "random skill" or "skills" (all).
Stat: Same as "skill".
Steal: Takes from an enemy's skill and gives it to this troop. The amount stolen is limited by what the target has.
Summon: Creates a new troop in the first available slot. The troop's level is determined by the caster's magic, with an upper limit of 15.
Undead: Any troop with a skull symbol on their card.
If a spell says it will do damage but doesn't specify to whom, the first enemy is targeted. A spell may also target something random or all items of a certain type. But there are many spells that allow you to choose a target. This is how you know when you'll see those options.
"Select an enemy": Appears whenever the spell says "an enemy". E.g., not "a random enemy".
"Select an ally": Appears whenever the spell says "an ally".
"Select a gem": Appears whenever the spell says "a/selected gem" or "a/any/selected [mana] color, or "a row and column".
"Select a column": Appears when the spell says "a/selected column".
"Select a row": Appears when the spell says "a/selected row".
(My invite code is Aldin.)