Here is a list of foes you will fight against in Star Wars Episode III: Revenge of the Sith.
The Droid Army
The CIS are a group that left the Republic to take over the universe and kill the Jedi. It all started when Nute Gunray wants to get revenge against Padme. The Trade Federation attacked Naboo and Nute Gunray was charged for War crimes. After that event when the Jedi trained Anakin the Trade Federation and the Dark Jedi Dooku struck back with more friends. The Jedi were about to be killed until the CIS Battle Droids realized that the Republic now have a new army the clone troopers. After various of wars the CIS are forced to shut down all droids and then Anakin just come on in to kill everyone and after that Anakin’s eyes are yellow.
The standard battle droids of the CIS are easy foes to beat but in numbers they are quite deadly.
The Super Battle Droid is a super version of the regular Battle Droid. More armour and weapons and bigger.
The MagnaGuard are elite robots that carry things that can’t be cut by lightsabers. Often guard Generals, the Separatist Council and Counts.
The Droid Starfighter and the Droid Trifighter are fighters that can outmatch even the ARC-170 and the Jedi Starfighters.
The droids will stop appearing as foes after Grievous's death.
Basically Clones of Boba Fett these clone troopers look all the same and dress the same and formed the Grand Army of the Republic. The 501st legion is the feared part of the Republic and Vader’s personal team. The troopers were cloned from Kaminonians by Sifo-dyas order to counter the Separatist overwhelming Droid army which threatens the Jedi. They battled first at Geonosis to save the Jedi from the Droids but after the death of Grievous Order 66 turned the Clones into Stormtroopers and obey Sith lords.
Clone Troopers start being foes after Order 66 but the 501st Legion will help out killing Jedi and later on become stormtroopers.
The Jedi Order
The Jedi were guardians of Peace for a long time until to many wars made them almost extinct except for Obi-Wan and Yoda who survived Order 66. The Jedi Master Sifo-Dyas requested a clone army to help protect the Republic and help the Jedi but several Jedi left the Jedi Order and that includes Dooku.
The Jedi will appear as foes during Order 66 but watch out for elite Jedi.
The Dark Side
The enemies of the Jedi, They went into hiding until Sidious made a plot to take over the universe. The Sith are all once Jedis until only Jedi Knights can be Seduced to the Dark Side. The Jedi get corrupted due forbidden love and that includes Anakins Mom death (Anakin started going to the dark side after his mom died) and fear of Padme’s death (He tried to get Mace to not kill Sidious but ended up in he accidentally slice off Maces hand instead of stopping his purple blade).
Dooku and Anakin will appear as foes in the game but mostly you will control Anakin during a few Jedi killing missions.