They seem to have developed their own hashing function, of the sponge family, called Curl - and are actually using the Westernelitz (oh jesus spelling, more space-efficient Lamport) signature scheme - it is a method of constructing a digital signature only from hash functions. Cool.
Not to argue semantics but its isn't a chain. It may have blocks (containing one transfer) but it is by no means linear. This lets the tangle achieve verification parallelization. Which is in contrast to blockchain’s strictly sequential, synchronous ledger.
Having said that, the positives about IOTA;
They seem to have developed their own hashing function, of the sponge family, called Curl - and are actually using the Westernelitz (oh jesus spelling, more space-efficient Lamport) signature scheme - it is a method of constructing a digital signature only from hash functions. Cool.