Short answers, pulled from the story.
EC Comics was founded by Max Gaines, the former editor of All-American Publications, initially under the name Educational Comics. When All-American merged with DC Comics in June 1945, Gaines retained rights to Picture Stories from the Bible and continued building the EC line independently.
EC originally stood for Educational Comics, the name Max Gaines gave the publisher when he planned to sell Bible and history comics to schools and churches. After Max Gaines died in 1947, his son William Gaines renamed the company Entertaining Comics.
EC Comics ended its horror and SuspenStory titles on the 14th of September 1954 after distributors refused to handle them following the creation of the Comics Code Authority. The Code prohibited the words horror and terror on comic covers, and Gaines refused to submit to its authority.
Comics Code Administrator Judge Charles Murphy demanded that EC Comics change the ending of the story Judgment Day, originally published in Weird Fantasy #18 in April 1953, because its central character was a Black astronaut. Bill Gaines and editor Al Feldstein refused and threatened a lawsuit; the story ran unaltered in Incredible Science Fiction #33 in February 1956.
Mad began as a side project for editor Harvey Kurtzman and became EC's most commercially durable title. When the Comics Code Authority and distribution problems destroyed EC's other comic lines in 1954-55, Gaines concentrated all resources on Mad, converting it to a magazine format that removed it from Comics Code oversight.
EC Comics employed a large stable of freelance artists including Johnny Craig, Reed Crandall, Jack Davis, Will Elder, George Evans, Frank Frazetta, Graham Ingels, Jack Kamen, Bernard Krigstein, Joe Orlando, John Severin, Al Williamson, Basil Wolverton, and Wally Wood. The company was unusual in allowing each artist to sign their work and publishing one-page biographies of them inside the comics.
EC Comics adapted roughly two dozen Ray Bradbury science-fiction stories starting in 1952. The arrangement began after editors Feldstein and Gaines plagiarized two of Bradbury's stories without permission; Bradbury's response led to a negotiated licensing agreement. Reprint collections of these adaptations include The Autumn People and Tomorrow Midnight.