You need at least one agent to build your code.
Deploy Windows agents to build Windows, Azure, and other Visual Studio solutions
Deploy Xplat agents to build Xcode, Android, Java, or other kinds of apps
If you are new to Team Foundation Server (TFS) and Visual Studio Online, you should use this new system. Most customers with experience using TFS and XAML builds will also get better outcomes by using the new system.
The new builds are web- and script-based, and highly customizable. They leave behind many of the problems and limitations of the XAML builds. For example, this new system embraces the diverse set of domain-specific languages (DSLs) that developers used to build the code on their dev machines. We expect this will reduce the chances of running into the "builds fine on my machine but not on the CI server" problem. For more details, see Overview of Team Foundation Build 2015.
In a few cases you might need to continue using XAML builds:
If you use Team Foundation Version Control (TFVC) (not Git):
Gated builds are not yet available.
Complex workspace mappings are not supported.
If you have heavily customized XAML builds and custom activities, you can continue using those builds until you are ready to port your business logic into scripts that can run in the new builds.
The new builds are based on a different architecture and run on a completely different system.
You can use both the new builds and agents alongside your XAML builds, controllers, and agents.