The definitions of the below terms are copied or are modified forms of the definitions from chapter 2 of the OpenCL 1.1 specification.

Compute Device
Compute devices are the target of computation to be off-loaded from the host CPU. Command-queues are created in OpenCL applications and are tied to a specific compute device. Internally, a compute device is a collection of compute units. OpenCL compute devices typically correspond to a GPU, a multi-core CPU, or multi-core DSP.
Compute Unit
A compute device contains one or more compute units. For multi-core devices, a compute unit often corresponds to one of the cores. A work-group executes on a single compute unit and multiple work-groups can execute concurrently on multiple compute units within a device. A compute unit will have local memory that is accessible only by the compute unit.
Command Queue
An object created by OpenCL APIs in the host application. A command-queue is created for a specific device in a context. Command queues hold commands that will be executed on that specific device. Commands to a commandqueue are queued in-order but may be executed in-order or out-of-order depending on the attributes specified during the command queue’s creation. A compute device may have many command queues associated with it, but a command queue will only associate with one compute device.
A kernel is a function declared in an OpenCL C program and executed on an Compute Device. A kernel is identified by the __kernel or kernel qualifier. Kernels are enqueued to compute devices through command queues.

When a kernel is enqueued to a command queue, the enqueue command specifies the number of work-items to be completed. For enqueueNDRangeKernel, the number of work-items is explicitly specified by the global size argument. For enqueueTask, the number of work-items is implicitly specified as 1.

One of a collection of parallel executions of a kernel invoked on a device by a command. A work-item is executed by one or more processing elements as part of a work-group executing on a compute unit. A work-item is distinguished from other executions within the collection by its global ID and local ID.

A collection of related work-items that execute on a single compute unit. The work-items in the group execute the same kernel and share local memory.
Global ID
A global ID is used to uniquely identify a work-item and is derived from the number of global work-items specified when executing a kernel. The global ID is an N-dimensional value that starts at 0 in all dimensions.
Local ID
A local ID specifies a unique work-item ID within a given work-group that is executing a kernel. The local ID is an N-dimensional value that starts at 0 in all dimensions.