I thought that blockchain was the only way to build distributed ledgers. Seems like there is a new technology: hashgraph. This article, Blockchain Just Became Obsolete: the Future is Hashgraph, helps you understand the difference between blockchain and hashgraph. In particular, this 10 minute video is very helpful.
Remember that a distributed ledgar is an application or a use case. Blockchain and hashgraph are technologies that are used to implement distributed ledgers.