Stellar (XLM) is a completely decentralized consensus platform. It is designed to support any type of currency. It has a built-in decentralized exchange that can be used to trade any type of currency or asset.
Lumens are the native asset (digital currency) that exist on the Stellar network and acts as a bridge in-order to facilitate multi-currency transactions. Transactions such as: sending money in EUR, and then receiving it in USD. XLM is the digital intermediary that allows for such a transaction to occur at a low cost.