<(comando)
A command's output dressed as a file — without touching the disk.
Process substitution hands over a command where a file is expected: <(cmd) becomes a name — /dev/fd/63 — that, when read, delivers the command's output. A diff between two outputs needs no temporary files.
The twin >(cmd) points the other way: whatever is written to the name feeds the command's input — tee >(gzip > log.gz) stores compressed while showing on screen.
$ diff <(sort ninho-a.txt) <(sort ninho-b.txt)
2c2
< besouro
---
> cigarra