The four races come with their own distinct sense of design, but really, they're largely interchangeable with each other. SupCom units are about as generic as it gets: you get tanks, bots, planes, ships, missiles.
The United Earth Federation or UEF are the Blue Squares. This your standard issue side of order and authority, and the one plays close to us humans. In fantasy terms they'd be the knights in armour everyone defaults to when they start out. The UEF has a square, blocky appearance and a straightforward design palette that's immediately readable to any newcomers to the game, or the genre. They have a bit of everything, and everything they wield is either average or above average at what it does. What they use in the first tier of weaponry is basically the same all the way through to the last, except each tier gets bigger, meaner and more expensive.
Tanks look like tanks, planes are either delta winged jets or helicopters, the boats look like modern naval shipping, turrets are boxes with tubes, and so on. One of the big distinguishing features of the UEF is its extensive use of assault bots: big metal men with big metal machine guns.
The Cybran are the Red Triangles. They're the classic opponents to the good guys - aggressive looking war machines with a slightly anthropoid bent; crablike, non-human mechanoids with lots of sharp edges and spikes. They'd be the equivalent of Tolkenian Orcs or the Soviets in other titles. Cybran are not forthright and honourable; why, they're devious and tricky, with an emphasis on stealth and brutal pragmatism. They conceal themselves under huge radar jammers, and render their navies practically invisible. They're one big weakness is a lack of decent force fields compared to the other factions.
The best way to use the Cybran is to take full advantage of their wide, generous anti-radar, and get into the habit of actively concealing your forces using cloaks and radar jammers. Cybran's tiny unarmed scout is a fantastic little guy who automatically cloaks when he stops moving; ideal for scouting the map and then surreptitously spying on your opponent (or heh, sitting on their metal deposits!). They can be revealed by the enemy's ACU or good radar coverage, but
The Aeon Illuminate are the Green Circles. Elegant, high tech Star Trekkie designs, and graceful curves definitely place the Aeon as surrogate space elves. Their units are lighter in health points and stealthy, with far greater mobility and "magic" (i.e. forcefields) than the other factions. They seem to have more useful hover tanks than normal, which can threaten not just land units but also challenge navies on the high seas and sweep across island maps with alarming speed. However, they do seem to have more specialised units that are very good in some situations, but awful in others. The hovers themselves have good speed, but lighter weaponry and a tendency to trip up on the smallest rock - or themselves. The Aeon seem to work best in multi-disciplinary packs.
The Seraphin are the Yellow Diamonds, the extra race added with the Forged Alliance expansion. Space elves have their old, elder gods, and by gum, here they are. The Seraphin are the alien race that the Aeon Illuminate befriended in the previous game, before getting blah-de-blahed out of existence by some arbitrary and forgettable bit of off-the-shelf nuclear apocalypsing that flashed past in the first game's opening cut scene. Or something. Well, they're baaack, and they've sworn death and destruction on the whole human race and tum-te-tum. Their look is that of floatring metallic crystals and funky metal teardrops with those nifty glowing rune lines we all know and love from properly accessorised elder gods. Units look pretty weird all round, and they're definitely the race for the hardened vets who know where everything is, and seek a fresh take on the game. The Seraphin are big on strange visual effects and hover technology. They also sport some seriously tricky, stealthy and shielded units as well.