In certain contexts, they mean the same thing, but there is some nuance!
An editorial is a periodical article that gives the opinions of the editors or publishers. Many US publishers label them as "Opinion."
Opinion pieces are sometimes editorials, but are often written by members of the community.
Letters to the editor are short (rarely over 200 words) and are meant to be published quickly in response to articles or news that publication recently put out. They're typically written by a reader of that publication, a general person.
Op-eds, sometimes labeled "Opinion" by places like the New York Times, are usually longer, and means "opposite the editorial page." They, too, are written by members of the community, not newspaper employees. They are often experts who have a particular insight into a topic. Often they submit their article for publication or their writing is solicited by the publication itself.
See the links below to more context and definitions.
