Long time Star Wars fan, cautious Wikian, mainly creating an account right now so that I can track the odd article.
Saved here for my own reference, in order that I have a quick and easy way of referring to the differences...
Form I: Shii-Cho "Way of the Sarlacc" or "Determination Form"
Basic and simple, the beginning point of lightsaber training. Focused on disarming without killing, useful in defending against multiple opponents rather than one-on-one, deliberate moves, 'clumsy bur relentlessly dogged'. Users could be prone to giving into the kill.
Form II: Makashi "Way of the Ysalamir" or "Contention Form"
Designed for lightsaber-to-lightsaber dueling, benefits a hook-handled 'saber, flowing moves, feints, economy of movement. Could be weak against blasters or multiple opponents, and against power-based offence.
Form III: Soresu "Way of the Mynock" or "Resilience Form"
Designed to counter blaster fire. Short, defensive movements - 'eye of the storm'. Required intense concentration, waiting on an opponent's mistakes. User avoided being struck, and could use acrobatics and aspects of other styles to this end.
Form IV: Ataru "Way of the Hawk-Bat" or "Aggression Form"
Acrobatic, 'Yoda's form', relied on speed, power and agility. Required extensive control of the Force for most elaborate moves. Best used against single opponents and in short battles.
Form V: Shien / Djem So "Way of the Krayt Dragon" or "Perseverance Form"
Shien - used against blasters. Djem So - used against lightsabers. Developed from Form III to provide more offence. Physical strength used to control opponent - concern raised among some Jedi. Defence immediately followed by attack. Wide, powerful strikes. Shien weak against one opponent. Djem So users could be immobile.
Form VI: Niman "Way of the Rancor" or "Moderation Form"
Jack of all trades? Contains aspects of all five other Forms. 'Diplomat's form'. No specific strengths or weaknesses. Covered all bases, so users had some understanding in all areas. Users were guided by the Force, continually adapting.
- Duel 'saber style, with one blade used to support the other. Offence and defence were available at the same time. Defence could be breached by a strong opponent due to inability to focus defence on a single blade.
Form VII: Juyo / Vaapad "Way of the Vornskr" or "Ferocity Form" Juyo was the most vicious form, chaotic, totally focused on attack and vulnerable to Force attacks. Bold, powerful and direct attacks often unpredictable.
Vaapad led through the dark side. Users had to relish combat and victory. Required constant use of the Force. Users fed the opponent's own darkness back into them. Fast movements gave the appearance of many lightsabers in play at once.
So that's what that thing in my toybox was...