|Developer||Epic Games, Inc|
|Publish on||Windows, iOS|
UDK is free for non-commercial, a fee and royalties must be payed for commercial use
Unreal Engine is the game engine that was used by Epic Games to develop the Unreal series, Infinty Blade, Gears of Wars and much more.
Being a tool for pros and big budget studios, UDK is not noob friendtly, meaning that you require a certain proieficiency in game development before starting to work with UDK. For example, the editor is very complex and can be overwhelming
UDK comes with tools for all needs to create a game: animation, Ai, audio, particles (called Cascade), in-game cinematics and cut scenes (called Matinee), the editor (above image), kismet (a visual scripting system), lightmass (lights and shadows system), terrain editor, networking (LAN and direct IP connect), nVidia physX's based physics engine, real time shaders, multi-threading rendering, UnrealScript similar to Java, Unreal Swarm a network distributed computing task system, UI with Scaleform.
The forum located at http://forums.epicgames.com/forums/366-UDK is a precious resource for learning and getting help. Also a good start for anyone new is the documentation section http://www.unrealengine.com/en/udk/documentation/ and if you coulnd't find the answer to your problem, there is always the good old Google or even better YouTube.