In your average, memory-protected environment, a "context" is a virtual address space, the executable contained in it, its data etc.| wiki.osdev.org
Protected mode is the main operating mode of modern Intel processors (and clones) since the 80286 (16 bit). On 80386s and later, the 32 bit Protected Mode allows working with several virtual address spaces, each of which has a maximum of 4GB of addressable memory; and enables the system to enforce strict memory and hardware I/O protection as well as restricting the available instruction set via Rings.| wiki.osdev.org
This article discusses x86-64 CPUs (AMD64 and Intel's equivalent EM64T implementation). IA-64 (Itanium) is really a different beast and not addressed here.| wiki.osdev.org
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The Global Descriptor Table (GDT) is a binary data structure specific to the IA-32 and x86-64 architectures. It contains entries telling the CPU about memory segments. A similar Interrupt Descriptor Table exists containing task and interrupt descriptors.| wiki.osdev.org