TES Construction Set (The Elder Scrolls Construction Set) is a smart and powerful software solution that allows you to edit and create any data for use with The Elder Scrolls IV: Oblivion.
Any data that goes into the game goes in through the Construction Set and is stored in data files (labeled either ESM or ESP files) that are read by the game.
The Construction Set allows you to create your own areas (towns, dungeons, islands, etc.) and populate them with characters, creatures, and stories. You can add new races, classes, spells, quests, potions, magic items, and anything else your mind can dream up.
You can also edit the data that comes with Oblivion. Don't like how fast everyone moves? Change the movement settings. Do you want Nords to start with better abilities? Want to rebalance spell costs? It's all here.
The most powerful feature of the Construction Set is the ability to create new data and stories, which are stored as plugin files. You can place your plugin on the Internet and have others play the quests and areas you have created. And since they are plugins, they add directly into someone's existing game without damaging the data already there.