whats the difference between developing and publishing. why do they have the same name
Developers actually make the game. The trees, rocks, people, quests, everything you see in video games are done by the developer. The publisher is responsible for marketing. They make the ads, get the money to where it needs to go, manage certain services, and make sure the game gets onto shelves.
The reason that the two have similar names is that they used to be synonymous. Bethesda simply made games, and they would leave it to other companies to publish. Oblivion was published by 2K, Morrowind was published by Ubisoft. Bethesda Softworks has always been a publisher, but I believe they were pretty limited in what they could do at the time, because publishing is a pretty big financial responsibility. EA, for instance, plans to spend $100 million on the marketing campaign for Battlefield 3 alone. Most companies don't have that kind of money to throw away and get their name out there. After many successful game releases, however, I don't think Bethesda Softworks has any worry about money, giving them liberty to publish all of their own games as well as the games of Zenimax's other studios, starting with the PS3 release of Oblivion. Steam is also a very useful partner because it helps save a lot of money on physical boxes and distribution, while protecting against piracy at the same time.
Unlike all of Zenimax's other studios, Bethesda Game Studios is "owned" by Bethesda Softworks. It's an in-house subset, and that is why they share the "Bethesda" name.