UnrealScript (often abbreviated to UScript) is the scripting language of the Unreal Engine and is used for authoring game code and gameplay events. The language was designed for simple, high-level game programming. The UnrealScript interpreter was programmed by Tim Sweeney, who also created an earlier game scripting language, ZZT-oop.
Similar to Java, UnrealScript is object-oriented without multiple inheritance (classes all inherit from a common Object class), and classes are defined in individual files named for the class they define. Unlike Java, UnrealScript is case-insensitive, and does not have object wrappers for primitive types. Interfaces are only supported in Unreal Engine generation 3 and a few Unreal Engine 2 games. UnrealScript supports operator overloading, but not method overloading, except for optional parameters.
By making the process of modifying the game easier, UnrealScript helped enable the growth of a large modding community around Unreal. This greatly added to the overall longevity of Unreal and provided an incentive for new development.
Read more about UnrealScript: Functions, "Hello, World" Example