Additive and subtractive
When you create a new map by going to File | New, you are presented with two options to create a map inside UDK: additive and subtractive. The most common and mostly used is the Additive mode.
Additive
Think of an empty space. In the additive method, you would be adding walls, models, and the environment assets into the empty space. A real life example of additive map creation would be constructing a building wall-by-wall.
Subtractive
Think of a solid rock. In the subtractive method, you would be cutting the rooms and environment out of this solid mass. Subtracting the solid rock, in order to have room, is like sculpting.
UDK basics covers the most essential tools and functions you need to know to get started with UDK. You'll be able to quickly jump into UDK and begin feeling comfortable using the most commonly used functions.