Fillings are typically used to remove decay and replace the tooth structure that's been affected by it. This filling covers the hole left by the decay. Typically, fillings are comprised of tooth-colored composite resin.
Crowns are meant to provide a complete structure to cover a tooth that is fractured or severely damaged and can't be restored with a simple filling. These are also always placed after a root canal.
Sealants are utilized in order to fill grooves in a tooth that can easily be affected by decay.