GDAX, a popular exchange run by Coinbase has added margin trading to the services it offers. While not available to everyone, authorized traders can lever their orders up to 3 times what they hold. Users can access margin trading for Bitcoin, Litecoin, and Ethereum. Margin trading, which is most common in stock markets, is a process by which you borrow money from your broker in order to sell or buy more stocks (or, in this case, cryptocurrency) than you can afford. It's a short term loan that allows you greater leverage when purchasing or selling an asset.
Because Coinbase is attempting to stay within the bounds of the Commodity Exchange Act, it is limiting this offering to mostly high-value institutional investors who must certify that they meet one of the qualifications to trade on margin. A full list of qualifications can be found here.
Currently, the exchange is not charging interest or fees on margin trading because they believe that consumer lending laws require specific license in order to do that which, apparently, Coinbase doesn't currently hold.