Element is a chat client. It's an empty piece of software for use with your own choice of server. Element is to a chat server, as Thunderbird is to an email server. It's basically a glorified IRC client. It contains no content of its own.

Parler was basically a curated, centrally run, Facebook-message-board-replacement for neonazis, antisemites, qanon conspiracy theorists, and the lunatic fringe of the alt-right.