kernel: A kernel is a small unit of execution that performs a clearly defined function and that can be executed in parallel.
work-item: A instance in which one kernel run.
work-group: A group of work-items. Work-items grouped into a work-group are executed simultaneously.
NDRagne: An index space.
OpenCL application: Host program running on host + Kernels running on Compute device.
In the global NDRange index space, work-item has a unique coordinate(global ID), and work-group has a work-group ID. In a local work-group index space, work-item has a unique coordinate(local ID).
Therefore, one work-item can be accessed by global ID or work-group ID + local ID.
With plaform model
Interaction between a host and an OpenCL device is caused by commands. The host uses command-queue to transmit commands to the device.