In computer science, a task context (process, thread) is the minimal set of data used by this task that must be saved to allow a task interruption at a given date, and a continuation of this task at the point it has been interrupted and at an arbitrary future date. The concept of context assumes significance in the case of interruptible tasks, wherein upon being interrupted the processor saves the context and proceeds to serve the interrupt service routine. Thus the smaller the context the smaller is the latency.
These data are located in:
Processor registers
Memory used by the task
On some OS, control registers used by the system to manage the task
From: http://en.wikipedia.org/wiki/Context_%28computing%29