The cordless drill gets its name from the simple fact that it is a power drill without a cord (it runs on batteries). They are commonly used for the same purpose as a normal electrical drill: drilling holes and driving screws.
In the past, cordless power tools have been underpowered, weak and heavy but with the introduction of Lithium-Ion batteries, these tools are just as powerful as their corded counterparts and in some cases, even lighter. Another benefit of a cordless drill using Lithium-Ion batteries is that the batteries are also capable of being recharged quickly, usually 30 minutes or less. Without the cord, a cordless drill is typically earlier to use and more convenient.
The above example of a cordless drill is the Milwaukee M12 Drill/Driver. It has a 12 volt Lithium-Ion battery and handle drill bits 3/8″ in diameter (larger if they have a reduced sized shank). It is a powerful, lightweight cordless drill capable of giving you years of use.