These are commands specific to auras. You can get an aura through Unit methods GetAuraObjectById(spell id), and use that similar to how you would use Unit or Player. For instance.

function OnCombat(Unit, event)
  local aura = Unit:GetAuraObjectById(1337)
  local spell_id = aura:GetSpellId()
  -- spell id would be 1337
  tostring( aura:GetCaster() ) == tostring( Unit ) -- would return true
  -- Just stick aura, in front of the command the same way you would Player or Unit

Function List

GetObjectType() Will return Aura if the aura is not nil.
GetSpellId() Returns the aura’s spell id.
GetCaster() Returns the object that casted the aura. Can be a Unit, GameObject, or Item.
GetTarget() Returns the target of the aura; the person who is currently affected by it.
GetDuration() Returns the duration in miliseconds.
SetDuration(duration) Sets the duration of the aura. The aura will be removed after the duration has passed.
GetTimeLeft() Returns the amount of time left until the aura expires in miliseconds.
Remove() Removes the aura & all of its events (duration, etc.,).
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.
GetVar(var [,subindex]) See above, but returns the value on success or nil on failure.
GetAuraSlot() Returns the slot that the aura is in. See Unit.h for meanings.
SetAuraSlot(slot) Sets the aura’s slot. See Unit.h for meanings.

The following 3 functions will be called from a Player or Unit, not an aura, and they deal with or return an aura object.

GetAuraObjectById(spell id) Returns an aura object for the spell id.
AddAuraObject(aura) aura is not a spell id. It is an aura object.

The following do not return aura objects, and are still called from a Player or Unit, not an “aura” object.

RemoveAura(SpellID) If the unit has the aura with the spell given, will remove it.
RemoveAllAuras() Removes all auras, positive or negative, from the unit or target.
HasAura(spellID) Checks if the target or unit has the spell aura specified.
RemoveAurasByMechanic(string, 1 or 0) Removes auras with the mechanic specified in string form, set to 1 for only the hostile auras, set to 0 to remove all auras with given mechanic type.
RemoveAurasType(type) Removes all auras with the given type, similar to RemoveAurasByMechanic.
AddAura(spellid, duration, temp) Adds aura with spell id, duration, and if it is temporary or not (true/false). See core for more info.
RemoveNegativeAuras() Removes every negative aura from the unit.
HasAuraWithMechanic(number) Returns true if the unit/player has the aura with said mechanic.
HasNegativeAura() Returns true if the player has any negative aura.
HasPositiveAura() Returns true if the player has any positive aura.
GetAuraStackCount(spell id) Returns the number of stacks the aura has.