Ethereum uses a variant of the GHOST algorithm, which was introduced in the paper Accelerating Bitcoin’s Transaction Processing: Fast Money Grows on Trees, Not Chains.
The Ethereum white paper mentions how they modified it:
Full detail is in their infamously difficult Yellow Paper:
I had some idea about the Bitcoin blockchain - also succinctly summarized by @tylerswift.
But the Ethereum material is all new to me. It at least sounds more interesting than "the least interesting".