This site is all about the Republic Commandos, who reside in the Star Wars galaxy.
In Star Wars Episode II: Attack of the Clones, we find out that the Republic (the governing democratic body of the Star Wars galaxy) is becoming increasingly corrupt and unstable. Thousands of star systems are threatening to leave and join a new 'Separatist' movement. It becomes clear that a major conflict is brewing. And that's when Jedi Master Obi-Wan Kenobi stumbles across an army of clones on the planet Kamino. The Kaminoans were dutifully creating an army of millions of clone troopers, all based on the genome of a bounty hunter called Jango Fett, but the Jedi Master who placed the order had not informed the Jedi Council (the governing body of the Jedi Knights, the guardians of peace and justice in the Republic). Despite the mystery surrounding their beginnings, the clone army couldn't have been ready at a better time, as a major crisis was about to occur on the planet Geonosis. That incident marked the beginning of a devastating galaxy-wide conflict known as the Clone Wars. Little did anyone know that the Supreme Chancellor of the Republic, Palpatine, was actually a Sith Lord (the enemies of the Jedi, bent on galactic domination) and he had masterminded the wars in league with his apprentice Count Dooku, who was the leader of the Separatists. The outward purpose of the Clone Wars was ultimately fictitious, but that made no difference to the average soldier on the front lines, who, like most of the galaxy's citizens, could not see how events were being manipulated. After Palpatine seized full control of the Republic and declared himself Emperor of a new Galactic Empire, the clone troopers eventually became the "stormtroopers" we see in the latter three Star Wars movies, Episodes IV to VI.
What exactly is a clone? Unlike regular living beings born from a male and female parent, clones are grown using the DNA of one donor only, and they grow to resemble that parent exactly. The Kaminoans were excellent cloners, and they could not only grow and train millions of clones at once, but they could even modify the base DNA to have certain personality traits expressed or not expressed. The clones in Star Wars are genetically modified to be less independent than the original host, Jango Fett. (As Jango was not a team-player, he would have made a poor infantry soldier.) In addition, growth acceleration technology is used to halve the time necessary for a clone to mature. Instead of it taking 20 years for a clone to grow, the Kaminoans were able to accelerate the process so a fighting force could be grown and trained in just 10 years. Unfortunately, the growth acceleration continued into adulthood, so the clones had a greatly shortened lifespan.
The Republic Commandos are specially trained clones. Genetically, they are identical to regular clone troopers - they all share Jango Fett's DNA, modified to make them more docile, and are growth-accelerated so that a full-grown army could be created and trained in just ten years. Yet the commandos were set apart from the beginning, arranged in groups of four called "pods". Each pod was assigned a training sergeant, most of whom were Mandalorian mercenaries specifically recruited by Jango Fett for that purpose. Numbering around 10 000 at the start of the war, the commandos took heavy losses in the first few months of the war - due to the fact that inexperienced Jedi Generals often deployed them as infantry - but once they were able to operate in their element as special forces, their enviable objective achievement rate of 90% proved invaluable to the war effort.
The commandos were given basic military training, but they also received special training to make them capable of performing various kinds of spec op missions. Republic Commandos were called upon for assassinations, hostage extractions, demolitions, reconnaissance, and counterinsurgency, and they were trained to be adaptable: they could use just about any kind of weapon on any terrain in any environment at any time.
Training with the same other clones every day caused the Republic Commandos to develop more distinct personalities than most of the rank-and-file clone troopers. Four-man squads acted almost like families, allowing certain preferences and characteristics to be exaggerated in the clones, which resulted in increased individuality and creative thought. The Kaminoans were skeptical of this at first - they were obsessive about quality-control of their 'product' and saw deviance of any kind as failure. However, these traits increased performance in exercises, so eventually they ceased their attempts to recondition the whole batch.
The Republic Commandos' basic weapon is the DC-17, a smaller yet more versatile and advanced version of the basic clone trooper blaster rifle, the DC-15. The DC-17, also affectionately called the deece, has different attachments that can be changed quickly in the field - the basic ion repeater blaster, the anti-armour rocket-propelled grenade attachment, and the sniper attachment. This versatility allows the commandos to travel light yet be prepared for any number of situations. Commandos also have higher spec armour than regular clone troopers, and a more advanced helmet HUD ("heads-up display").
For information and summaries of media the Republic Commandos have appeared in, please see the Sources section.
Republic Commandos @ StarWars.com Databank
Republic Commandos @ Star Wars Wiki