SF armies come in several distinct flavors, which will determine their role in the story and their capabilities:
Army
The regular armed forces of a nation - trained, armed, and in large numbers. Will be able to field most of the units of the Standard trope, and take on other Armies. Or at least in theory - sometimes the budget gets cut, the Generals are a bit too wacky, or their enemies are a bit more powerful than expected.
The term "Army" here (and the trope title) is a misnomer: it could be loosely applied to any planetary force, even if they're using air craft and oceanic ships. It does serve its purpose in informal conversation, a common term used by others to refer to any military. However, if the characters are military personnel, the term should refer to a specific armed branch.
Organized along military lines, the Army should follow regulations and procedures. While fiction does take liberties with protocol, though it will affect how seriously the audience will take them. For example, a serious work should present rules and regulations as a vital part of military life. Inversely, the lack of enforced regulations suggests an undisciplined force. However, if the story is humorous, the lack of discipline (or strict adherence to it) can be played for laughs.
The vanilla of SF armies, these can take on a wide range of roles: the good guys, an obstacle to our heroes, The Cavalry , or even the bad guys. The writer can find many uses for them, depending on what they wish to write.
Alternatively, the Army could consist entirely of robots or monsters. With their will controlled by their masters, this army does not follow military traditions. They may not be sentient at all. Rarely a force for good, they're always antagonists.
Corporate Army
Will frequently show up in Cyberpunk. Large force of well-armed guards and Elite Mooks controlled by an economic entity, with a dash of black ops to take care of dirty work. Not meant for battle against regular Armies, it's main goal is to ensure and protect profit. The preferred choice of the Megacorp or Neo-feudal aristocrats - cheap but effective for their needs. Typically target practice for the good guys.
A corporate installation out in the boondocks, where real assistance might be a long time in coming(or nonexistent), may have heavier elements available, depending on how much trouble is likely to come their waynote
Sometimes Corporations will be powerful enough to field most (if not all) of a Standard Sci-Fi force. In this case, it's just another Army (see above), only answering to a MegaCorp instead of a nation — unless it's One Nation Under Copyright. But before reaching that point, most corps will simply hire Mercenaries instead.
Resistance
Rebels who are fighting for "Freedom", though what that really is depends on what their goals are. Lacking equipment and training, they must defeat their foes using their wits, and conducting guerrilla warfare and spying. Their forces will consist mostly of Infantry, stealing or salvaging any weapon they can. Often seen as good or beneficial, unless the heroes are fighting them. More cynical works featuring them may point out that "noble resistance" and "terrorism" can often be a matter of perspective.
Mercenaries
Hired to fight in others wars, mercs are highly trained and ruthless soldiers. The difference between a Corporate Army and these guys are that Mercs are meant to be used in the battlefield. As such, they're fully equipped and expert forces, with their own artillery, air support, and tanks. Maybe even logistics support.
Like any other business, Mercenary companies have a distinct image and a reputation. Their Company has a well-known and creative name, their combat prowess is known through the galaxy, and their uniforms and insignia are unique. It is also an image that has to be upheld - for if they fail too many times, the mercs have nothing to sell.
Mercenaries don’t fight purely for money (though they prefer to get paid). They’re loyal to someone, often their own brothers in arms. Tend to focus on ground forces, mostly armor and infantry. Even as protagonists, don't expect them to act like heroes. With no home to return to, their survival depends on doing whatever is needed to win.
Special Militia
Irregular, fantastic armed force (typically heroes of justice) backed by a benefactor who may or may not have good intentions. Lacking numbers, this private army uses high-technology to even out the odds. Often they're the only people that can deal with a threat. Members tend to be either ex-military or civilians, and the group lacks military traditions. This, along with their eccentricities, may explain their informal nature. Expect Exotic units (giant robots are popular), color coded uniforms and symbolic motifs.
