The spells, just like unit cards, are marked with a corresponding symbol and color. There are a few types of spell cards, namely: Common, Rare and Epic. The golden color has been reserved for Champions cards, so it's unavailable among spell cards.
In Legends of Runeterra, spells are divided into Slow, Fast and Burst. Each one of them is useful under certain conditions and fulfills a different task. It is worth knowing the characteristics of each spell in order to use it properly during a duel.
Slow spells are relatively slow - this means that their effect is not immediately triggered and they will take effect after one turn. Most importantly, Slow spells cannot be used during a fight so they are useless during an attack.
Fast spells are much faster and can be used during the fight as well as out of it. Most importantly, after casting such a spell, the opponent can react to it and use his own counterspell to cancel its effect.
Burst spells are the fastest, and their effect is immediate. This means that the opponent has no way of reacting to such a spell. Most importantly, after using the Burst spell, your turn doesn't end so you can still do some extra action.