The extensible SDK provides the yocto workflow without the need to first set up a full yocto build environment Setup and Building, as this is already included in the eSDK.
The eSDK must be installed in the same way as the SDK and before using it, you must run its source script. After this you rely on devtool.
The official yocto documentation can be found here
There are 4 different ways to develop software using the yocto project:
Use a full build, see Getting Started with Yocto and C/C++.
Use an SDK, see SDK.
Use the yocto workflow with an Extensible SDK, see below.
Build natively (on the target), see below.