Why are there 11 races? Why not 10 or 12? Well, because the game’s creator stopped at eleven, and the game has been perceived to be quite well-balanced since then (with minor tweaks over time). Ideas for a 12th race have often been proposed, but never implemented (edit May 2017: Planets.Nu has implemented a new race called the “Horwasp” At the time of this edit, it is still in alpha testing). So… (in normal games) choose from these eleven:
The Federation: A torpedo-based race (mostly) with two terraformer ships, and strong medium-to-large warships. Their flagship is the Nova Battleship, which is the 2nd-strongest battleship in the game, although it ranks only 8th overall when compared against carriers. The Feds have 200% money income but a 70% mining rate, and the ability to refit existing ships with upgraded parts. The Feds also have rapid-launch fighter bays, a shield-recharge bonus, and their weapons will always fire as if the ship is undamaged. All of their advantages together make them favourite allies for many races, although the Feds are not as strong on their own.
The Lizard Alliance: A torpedo-based race with many economic advantages, a 30:1 ground attack advantage, +50% ship damage resistance, and cloaking ships (cloakers). While their “big” ships are small compared to many other races, their fleet is very well-balanced, with useful small, medium, and heavy warships. Because any lizard-owned ship can take 150% damage before being destroyed, the Lizards often command most of the allied fleet in close alliances.
The Birdmen: this torpedo-based race has the best cloakers in the game, including the only cloaking battleship (the 3rd-best battleship in the game, and 9th overall). While they have no economic advantages, they do have a Super Spy mission that can reveal the exact nature of a planet’s defenses before they strike, and can even change the friendly codes of enemy planets, to steal resources or bypass minefields. Because their Resolute-Class Battlecruisers are so strong against starbases, the Birds are probably the strongest early-strike race in the game, capable of taking out an enemy home world by Turn 10, without even revealing their intent before it’s too late…
The Fascists: also a torpedo-based race, with mediocre ships that include some cloakers and a medium-sized battleship. They boast the dreaded Glory Device ships, which can be set to explode, to damage all enemy ships in the area. Their ships are immune to planet attack, and can also Pillage planets to kill inhabitants and take their money. More recently, the Fascists have been frequently granted the double-beams upgrade (depending on host settings), where their beam weapons recharge twice as fast, which makes deploying carriers much more expensive against the Fascists, as their ships will shoot down a great number of fighters in every battle.
The Privateers: as their name implies, this race steals from others to gain power. The “Pirates” have very weak ships, but can rob other ships of fuel, and even commandeer the ships themselves! They also have three ships with Gravitronic Accelerators, including the Meteor Class Blockade Runner, which all move at double-speed. Playing the Privateers requires great planning and care, but a properly-played Privateer is a race feared by all players.
The Cyborg: This race’s defining ability is the ability to assimilate natives and convert them to colonists. While their small and medium warships are weak, they boast the dreaded Cube ships: the strongest battleship in the game (The Annihilation), and the second-best heavy carrier in the game (The Biocide). Furthermore, their Firecloud ships can open warp “chunnels” between themselves, allowing long-distance, instantaneous travel for multiple ships. They can also seed planets using their hyper-jumping Probe, assimilating natives in distant clusters. However, they are easily defeated early game, as all their ships except for their Cubes are very under-powered. More unfortunately, the Cyborg do not make friends easily, and are frequently targeted by other players because of how dominant they can be in the late stages of the game. However, it is also been told that a single probe with a single clan can re-build an entirely new Cyborg empire…
The Crystal People: as their name implies, this race is a silicon-based race, and they favour very hot planets. This racial trait gives them the ability to colonize many planets that would otherwise only support very small outposts of other races. However, their main specialty is their fuel-draining web mines, which have a high chance of latching onto a ship moving through them, and while their damage is light, the webmines drain fuel from the target ship. If the ship is caught in the web for too long and rendered fuel-less, the Crystals can send a ship to capture it! Otherwise, their battleship and carrier are fairly small, and their combat tactics rely on stealing ships from others. They are known as a very defensive race, and are most feared by cloaking races, whose cloaking is no defense against their webs. The Crystal People are weakest early game, before they can deploy massive fields of web mines.
The Evil Empire: this race boasts the largest heavy carrier in the game (which looks very much like a Death Star). They also have the dreaded Super Star Destroyer, which can send down agents to take over a planet at the cost of very few ground troops. In addition, they have both a Hyperspace Probe for scouting, and the Dark Sense ability, which can detect enemy clans regardless of any defenses they may build; however, they are for some reason unable to detect the Rebels at all. Like the Robots, they only have one torpedo ship for mine-laying, and although they cannot build fighters in space, their starbases auto-build five (5) fighters per turn. Contrary to their “evil” name, they are highly-dependent on allies to survive until later in the game, and must generally be savvy diplomats.
The Robots: a machine race, with some climate tolerance to go with it. They are most well-known for their massive carriers (3rd, 6th, and 13th best ships in the game) and superb mine-laying ability: they lay 4X as many mines for the same cost as other races. Unfortunately, like the Empire, they have only one ship that can lay mines, and it is not particularly strong in combat. Another large advantage of the Robots is that they can make fighters cheaply, within the cargo holds of their fighter ships. Because of their various advantages, the Robots do not fear any race in particular, and are quite strong both mid and late game, and their massive minefields usually allow them the time to develop, as even fast-striking races find it difficult to attack them early-on. The Robots’ one weakness is that their economy must be managed carefully, as they tend to use a lot of minerals and fuel to build and move their massive ships.
The Rebel Alliance: this race was originally modeled after the Rebel Alliance in Star Wars (TM). They have the one hyper-jumping ship in the game with a decent cargo hold (the Falcon), and also a decent array of both medium ships and the 4th-ranking heavy carrier. Similar to the Robots, they can make fighters in space. They have high tolerance to arctic worlds, and also the dreaded Rebel Ground Assault mission, where they can do great damage to a planet’s structures and population.
The Colonies of Man: this race has multiple, small advantages that add up to quite a powerful race. They have decent medium warships, and while their heavy carrier is the weakest of the heavy carriers in the game, it still ranks 5th overall. They have the Cobol, a ship that can make fuel using its ramscoop, the Lady Royale, a gambling ship that generates money, and the Aries, an inexpensive ship that can efficiently convert any mineral into fuel. Like the previous two races, the Colonies can make fighters in space. Last, they are the only race that can use their fighters for mine-sweeping, making minefields nearly useless against them.
Next, I recommend reading:
There are many more excellent overviews of the races in planets, written and available on other sites.
At Planets Magazine, which is attempting to become THE source for planets documentation, there are these articles:
“Who to kill, who to befriend” – gives a decent rundown of the races, their counters and their friends / enemies.
“What Race Should I Play?” – a beginner’s quick rundown of the race abilities and styles.