Note: Neutral/No Element has no strengths or weaknesses.
When a spell goes in direct strife again another (typically via Offense v. Offense, or Offense v. Defense), the Element it has comes into heavy play. If a Fire spell were to go against a Plant spell, the damage of the spell is doubled against the target (be it defense or offense). In turn, if a Plant spell went against a Fire spell, the damage of it would be reduced to half.
The majority of elements fall under one of these three categories, even if it is by a different name. If you do select a different name, make sure to include in parenthesis that element it is using (to determine it's strengths and weaknesses).