Hashgraph

Hashgraph is a distributed ledger technology that uses directed acyclic graphs to create an asynchronous Byzantine Fault Tolerance (aBFT) consensus algorithm. Hbar (HBAR) is the native cryptocurrency of the system. Unlike blockchains, Hashgraph does not use miners to validate transactions, and instead uses a "gossip about gossip" protocol to create directed acyclic graphs for time-sequencing transactions without bundling them into blocks. 
Hashgraph was developed in 2016 by the American computer scientist Leemon Baird, the co-founder and CTO of Swirlds, which holds several patents covering the hashgraph algorithm. The hashgraph concept has been described as a more advanced successor to blockchain technology. 
Concept.
Hashgraph has been described as a continuation of idea of blockchain or an alternative to blockchain, which attempts to provide speed, fairness, low cost, and security constraints. The Hedera white paper co-authored by Baird explained that "at the end of each round, each node calculates the shared state after processing all transactions that were received in that round and before," and it "digitally signs a hash of that shared state, puts it in a transaction, and gossips it out to the community."
Hedera Hashgraph.
Hedera Hashgraph is a public distributed ledger based on the Hashgraph algorithm. Hedera Hashgraph is developed by a company of the same name based in Dallas, TX, which in 2018 raised $100 million at a $6 billion market cap. The company was co-founded by Leemon Baird and Mance Harmon.
Criticism.
It has been claimed that hashgraphs are less technically constrained than blockchains proper. Cornell Professor Emin Gün Sirer notes that “The correctness of the entire Hashgraph protocol seems to hinge on every participant knowing and agreeing upon N, the total number of participants in the system,” which is "a difficult number to determine in an open distributed system.” Baird responded that “All of the nodes at a given time know how many nodes there are.”