An old article I write up on lightsaber forms that I thought I'd go ahead and repost here. I've recently added the Fast-Medium-Strong Styles as well, though variations like Shien and Jar'Kai are still missing.
Form I Shii-Cho: would be fair in offense and defense, it would use strength and thus tire the user. It's simple thus it allows the practitioner to focus elsewhere (force attacks, precognition, strategy). Its users sought to disable their opponent's weapon. The key is not in its technique but in its freedom it allows the user. To be deadly you use the most of your "free attack" that the form allows you. All other forms require significant focus and consequently this gives Shii-Cho its advantage. It is also a good fall back and foundation.
Form II Makashi: has a good offense and defense, coupled with excellent endurance, allowing the user to wear down his or her opponent. Its linear style makes it vulnerable to multiple opponents or attacks from several directions. This, together with its demanding focus, allows for surprise, blind-siding attacks. Makashi is superior in technique versus Shii-Cho, Niman, and Djem So. Against Soresu or Ataru it proves frustrating. Vaapad is a challenge and along with Djem So and Ataru, poses to disrupt Makashi's balance. This is its weakness. A disruption in their balance throws off their technique and weakens their defense making them vulnerable.
Form III Soresu: is the ultimate in defense and endurance. They try to outlast and tire their opponent into a mistake. Their patience, however, is their undoing, along with their focus. Break either and an opponent can go in for the kill. The best methods to do so include taunting and other psychological attacks. Another method would be distraction.
Form IV Ataru: uses acrobatic and light, fast strokes. If you can take away their mobility you can defeat them. Strike for the legs and "enclose" them with in the environment. Ataru requires acrobatics, thus they tire. It can out maneuver Shii-Cho, Makashi, and Niman. To defeat Soresu they must evade and erode their opponent's patience. They must attack just out of reach and force their opponent to go on the offensive. Against another Ataru wielding opponent it becomes a contest of the force. Against Djem So they must not engage their opponent because the strength of Djem So will disrupt Ataru's mobility. Vapaad is similar, by evading their attacks a Ataru user can disrupt their "joy of the fight."
Form V Djem So: has a strong offense and defense. Both require strength so it tires the user out. The focus is offense so the user can also open themselves up to an attack by loosing focus of their defense. The key is control. If they loose control they loose all sense of thought, yet sometimes this can work in their favor by enhancing their connection to the force. A battle frenzied opponent can easily be out-maneuvered and cut down, but one guided by the force is unstoppable. Djem So easily defeats Shii-Cho, Niman, with effort Makashi and Ataru. (Of course this is by looking purely at technique. For example, a Shii-Cho's defense would crumble against Djem So's offense, but Shii-Cho could use a force move followed by a counterattack to beat Djem So.) Soresu, on the other hand, proves frustrating. Again they must break Soresu's focus and patience, thus a Djem So user must act smartly. Vapaad is stronger and has better control but weaker defenses. Djem So must remove their "joy of the fight" through evasion and defense, yet they must be careful of Vapaad's unpredictability.
Form VI Niman: enjoys the best of everything and the worst of nothing. The strength of a Niman practitioner is just how far they have studied each form. A true master is an expert in each form and thus able to use all advantages in their favor. However, most study it only to the extent of being moderate in all forms, thus easily overwhelmed by a skilled opponent using advanced techniques and sets. A true master, on the other hand, has a very different weakness. For one, he must choose different forms, thus he must take time to "think" about which he wants to use with what moves. Confusion works well against a "thinking" opponent. Either use the weakness of his or her current form against them or make them stumble or hesitate in their decision. Masters must guard against this by practicing the "fluidity of transition."
Form VII Juyo/Vaapad: Juyo is supposedly an incomplete form. It is the attempt to take Djem So to the next level. They use their emotions in a controlled response to make each blow a critical strike. They use seemingly unconnected movements to throw off their opponent's guard. Their weakness is their control. By loosing control they loose their intense focus they need to perform their "unconnected" strikes in a fluid manner. In essence their defenses drop and remember, they were not strong to begin with. Vaapad takes it even further and "completes" the form. Through Vaapad even your defenses do not suffer, but for the Jedi it is an easy path to the dark side. However, it is also very effective against dark side users. It can channel their opponent's powers against them thus fueling your attack. Juyo/Vaapad easily defeats Shii-Cho, Makashi, Ataru, Djem So, and Niman. Soresu is a challenge as usual and has already been covered. Niman and Shii-Cho are obvious, they're out-matched. Djem So lacks the control and is a notch weaker. Makashi can be off balanced easily enough. Ataru's defeat merely requires making contact. Vaapad versus Juyo would go to the more "complete" form, especially if Sith Juyo vs Jedi Vaapad. A Vaapad vs Vaapad becomes a contest of control. When Jedi vs Jedi (for whatever reasons) of Jedi vs Sith, the Jedi must guard against falling to the dark side. However, if he/she were to give in to the dark side without hesitation, without loosing step, then he/she could still win and perhaps gain an advantage by shocking their opponent. (Note: not literally shocking, hence no force lightning intended.)
Fast: Fast Style focuses on speed and endurance. The short, quick strikes allow the user to outlast their opponent without tire and works well with acrobatics. Fast Style is also very effective against multiple opponents and blasters. However it easily overcome by the contact of a stronger styled opponent, thus Fast Style practitioners must be careful to avoid blows with more powerful opponents.
Medium: Medium Style is very similar to Niman in that it is a jack-of-all-trades form and excels out nothing while being weak at nothing. Yet unlike Niman, Medium Style also has a core foundation in Shii-Cho which gives the user an easy fallback form. Medium Style is best used against weaker or inferior opponents, and stands little change against skilled swordsmen of any other form.
Strong: Strong Style is a modernization of Djem So focusing purely on power without any regards to speed or defense. It pours all its energy into a brutal offensive designed to simply overpower its opponent. However, the slow strikes of Strong Style leave the user open to counterattacks.
Overall people learn several forms and master one or sometimes all. Whether anyone chooses to master only several is an interesting question. A good swordsmen doesn't adhere to a form, but learns his favorite techniques and creates his own using extensive variations, thus adding a new, powerful element: surprise. The unknown is the most powerful factor on the battlefield.
Most of my data was compiled from Wookieepedia but I came up with most of the advantages/disadvantages and tactics elements myself.
Marks of Contact
The following are named lightsaber strikes at specified limbs.
Cho mai: to try to cut off an opponent's weapon hand
Cho mok: to try to cut off an opponent's limb, whether it be leg or arm
Cho sun: to try to cut off an opponent's weapon arm
Sai cha: to try to cut off an opponent's head
Sai tok: to try to cut an opponent in half
Shiak: to try to stab an opponent, usually a limb but sometimes their chest
Shiim: to try to inflict a minor grazing wound on your opponent to open their defenses.
Sun djem: to try to remove an opponent's weapon either by dislodging it with a lightsaber or by physically attacking the opponent.
Mou kei: a lethal circular attack that removes all of an opponent's limbs