The Lightsaber Combat Forms
The basics of lightsaber combat comprised of attack, parry and bodyzone strikes. It is used primarily for training new Jedi in the use of the lightsaber.
The perfect form for lightsaber-to-lightsaber combat and studied by almost every Jedi. Form II maximizes precision whilst minimizing movement so as not to tire the dueller. Darth Tyranus (Count Dooku) both preferred and mastered this form. Sith Lords expecting to combat Jedi studied this form carefully as it proved to be very effective in defeating Jedi knights.
This form was developed specifically to counter the threat from laser blasts. As laser blasters spread throughout the galaxy Jedi had to learn to deflect/reflect laser blasts whilst offering the smallest possible target area. From this Form III was born. After the death of Qui-Gon Jinn this form was adopted and studied carefully by Obi Wan Kenobi. Many regard a true master of Form III as almost invincible. A truly defensive form.
The most acrobatic of all the lightsaber combat forms. Practitioners of Form IV took lightsaber combat to incredible new levels with amazing displays of physical prowess that were beyond anybody else. Watching a Form IV Jedi in battle would be difficult because of their ability to leap, spin, jump and fight in all 3 axis. Master Yoda was a master of Form IV. Other practitioners were Qui-Gon Jinn and Obi Wan Kenobi.
As the demand on the Jedi to maintain peace and order grew a more aggressive Form was needed for use in combat. Form V was designed to not only deflect laser blasts but also deflect them back on the attacker. Strength and strong lightsaber attacks feature heavily in this form which was favoured more by some Jedi than others. The morality of using an enemys weapon against them didn't sit well with some Jedi knights.
A combination of Forms I, II, III, IV and V. Often referred to as the "diplomats form" this particular style of combat relied less upon combative movement and more upon allowing Jedi time to study diplomacy, galactic politics and their Jedi perception. Considered a lesser form by some Jedi masters it proved to be very popular amongst many Jedi knights. To truly master Form VI will take over 10 years of intensive study.
The ultimate form of lightsaber combat which combines the raw strength of Form V with the graceful movement of Form IV. Only advanced Jedi masters such as Mace Windu can use this form effectively due to the huge emotions involvedin wielding it. Such is the emotional involvement that many Jedi refuse to study Form VII because it comes too close to the Dark Side of the force. Force assisted jumps and other movements feature heavily in Form VII as do series of sudden, seemingly unrelated, movements. A form VII practitioner will appear utterly calm on the surface but inside is a seething cauldron of emotions.
There is also Form Zero but very little is known of the form itself. This form basically teaches how to wield the lightsaber without igniting it. Or in other words - only ignite when your cause is just and the situation requires it. In the words of Felanil Baaks, Jedi Artisan and Teacher - "You must understand when to ignite and when to ignore. That is Form Zero."