Get (Return) Methods

GetCaster() Returns the caster of the spell. Can return a Unit, Item or GameObject.
GetEntry() Returns the entry ID of the spell.
GetSpellType() Returns the type of the spell.
GetSpellState() Returns the state of the spell.
GetTarget() Returns the target of the spell if the spell has a trigger effect. Can return a Unit, Item or GameObject.
GetPossibleEnemy([range]) Returns the GUID of a possible unit enemy of the spell. Range is optional.
GetPossibleFriend([range]) Same as above but for a friendly target.
GetObjectType() Returns Spell.
GetVar(var [,subindex]) See SetVar(var [,subindex], value), but returns the value on success or nil on failure.
GetCastedItemId() Returns what item ID cast the spell.

Modify/Set Methods

SetVar(var [,subindex], value) var is a string referring to a parameter of Spell. subindex is optional; used when the variable you are setting has sub indexes. value is what you want to set it to. Returns true on success, false on failure.

Is/Has (Boolean) Methods

IsDuelSpell() Returns true if the spell was cast in a duel.
CanCast() Returns true if the spell is castable.
IsStealthSpell() Returns true if the spell grants some kind of stealth.
IsInvisibilitySpell() Like IsStealthSpell() but for invisibility.
HasPower() Returns true if the caster has enough power to cast the spell.
IsAspect() Returns true if the spell is a hunter Aspect spell.
IsSeal() Returns true if the spell is a paladin Seal spell.

Uncategorized Methods

Cancel() Cancels the spell.
Cast(check) Casts the spell. Set check to true if you want to check if the spell is castable, or false to force cast.
Finish() Finishes the spell, used post-casting.
ResetVar(var) Resets the specified var to the DBC original. Returns true on success, false on failure.
ResetAllVars() Resets all of Spell’s vars to the DBC originals. Returns true on success, false on failure.