Batch. An SQL batch is a set of one or more Transact-SQL statements sent from a client to an instance of SQL Server for execution. It represents a unit of work submitted to the Database Engine by users.

Task. A task represents a unit of work that is scheduled by SQL server. A batch can map to one or more tasks. For example, a parallel query will be executed by multiple tasks.

Windows Thread: A windows thread represents an independent execution mechanism.

Fiber. A fiber is lightweight thread that queries fewer resources than a windows thread and can switch context when in user mode. One Windows thread can be mapped to many fibers.

Worker Thread

The worker thread represents a logical thread in SQL Server that is internally mapped (1:1) to either a windows thread or, if lightweight pooling is turned ON, to a fiber. The mapping is maintained until worker thread is deallocated either because of memory pressure, or if it has been idle for long time. The association task to a worker thread is maintained for the life of the task.