Unlike Bitcoin which uses a blockchain architecture for maintaining it’s ledger, IOTA uses the ‘Tangle’ which is a Directed Acyclic Graph, known as a DAG.
This Tangle solves both the scalability and transaction fee issues faced by Bitcoin (And most cryptocurrencies) by requiring the Sender in a transaction to perform a kind of proof of work which approves two transactions.
This removes dedicated miners and makes the system fully decentralised.