With its 80286 processor, Intel introduced a new feature: protected mode. All successors of the 80286 share this feature. Although they are able to operate in real mode like a 8086 processor, modern operating systems require protected mode. The 80386 processor with its 32bit protected mode is the grandfather of today's protecded mode systems and will be taken as an example to all protected mode operations.
A complete explanation of protected mode operation would be too much for my small page. Have a look at Intel's programmer's reference guides for a detailed explanation. Here are the major advantages of protected mode:
The main problem with protected mode is its completely different appearance to system programmers. If you are a DOS programmer, forget everything you learned about system programming! (OK, keep the PCs port adresses in your mind...) Close your eyes, take a breathe, open your eyes and explore the magic of protected mode programming.