Elements are titles for a set of Spells that share a common theme. Every element has 5 Spells in its spell set. Upon obtaining every element, there would be 155 accessible Spells (160 including Sans) in the game that you can choose from!
At the moment, there are now 31 Elements (32 with Events included). Each Element has its own unique power and effectiveness, each Element has its own move set appropriate to its theme. Elements are bought with Diamonds and moves are unlocked with Shards. The User can have their own personal skills to have a unique combo, as well as having different elements deal more damage than others. Elements costing more Diamonds tend to have Spells that deal more damage. However, the quality of an element, however, does not define the player itself.