Whenever the programmer attempts to access a new memory segment (by loading a value into the respective segment register), the processor reads the indicated segment descriptor from memory into a special segment descriptor cache register within the processor. From that point forward, the processor has instant access to the base address, length and other attributes related to that segment.
There are three types of descriptor tables:
Global Descriptor Table (GDT). Describes segments of memory that are global to all programs. There is only one GDT.
Local Descriptor Tables (LDTs). A particular OS may not implement any LDTs, only one LDT for all tasks, or a separate LDT for ...