Worlds are classified in many ways. The world tends to be classified by terrain, however, modifiers do exist. So, while it is perfectly acceptable to just say Oceanic world, sometimes one will encounter Civilized Ocean World, which indicates a world both that is both mostly water and has a firmly established presence of some space-faring civilization.
Arid worlds are much like desert worlds, though they have oceans, albeit small ones. They may have rivers and lakes, but much of the landmass is dry and lifeless, with small oasises of water.
Carbon worlds are worlds that are made mostly of carbon. Hydrocarbon oceans, diamond continents, these worlds are extremely valuable to anyone who possesses them. A single carbon world could produce enough laser rifles with the diamonds found on them to arm an entire galaxy, and the oceans of them could fuel the armies of mechas and tanks that often find themselves fighting for centuries.
A desert world is a world without oceans. Water may exist in springs or rivers, however, they are few and far between. Often, the entire planet is covered in sand or stone. If there is ice at the poles, it is temporary, lasting a few months at most.
A frozen world is, predictably, frozen. They have oceans, perhaps, under the thick ice, but there is nothing on the planet that is not snowy, frosted, and cold. Sometimes, life can still survive on the surface, and there may be places where the ice does not dominate. For example, the Frozen World of Iatsho was inhabited by a humanoid race known as the Kalisho, who lived near geologically active areas, until the DREL bombarded the planet in their Crusade of Purity.
Gas Giant worlds are worlds with an incredibly thick atmosphere, often dozens or hundreds of times the size of Earth. They often have lots of moons around them, and vast storms rage for decades or centuries.
A jungle world is one that has a thick atmosphere, very large biosphere, and is generally humid. Much, if not all, of the landmass of the planet is covered in humid rainforests, with frequent downpours. These rainforests can range from truly tropical ones, to forests akin to redwoods, cold yet wet. Oceans on these planets may cover 50-80% of the planet, and are often filled with just as much life as the forests.
Mixed worlds are combinations of several worlds. One terrain type my dominate, but there is a large mix of terrains that change over time. Earth is an example of a mixed world. They tend to be geologically active.
Magma worlds are extremely active geologically. Volcanos, lava flows, earthquakes, and other such things are extremely common. Any oceans on it will be hot, and any life on it will be incredibly accustomed to heat. There are patches where lava does not flow, and there are places that are less active than normal. However, the magma world is defined by chaos and lava.
A planet that is mostly ocean, 80-100% of its surface is covered in water. Much of the land is volcanic islands or thin lines of islands.
A world comprised mostly of flat, open areas with little variation in elevation. These planets tend to have weak tectonic processes. Oceans make up 30-75% of the world. Tree-like organisms may or may not exist.
A toxic world is a world that is filled with chemicals normally toxic to life. In addition to this, many toxic worlds have conditions that make life even harder. Venus, for example, is a toxic world with its clouds of acid and heat of hundreds of degrees. Most carbon worlds are also toxic worlds.
Methane and Ammonia worlds
Methane and Ammonia worlds are cold planets with "hydrospheres" consisting of methane or ammonia. These planets are brutally cold, but may support life that is utterly alien to most other worlds, with their biology being based around the use of their respective gases as a liquid solvent.
Planets are often classified, in addition to their terrain, with their habitation and purpose. For example, the planet of Chochai is both an arid world and a prison world, as it has little water and is covered in prison systems for the purpose of seperating people who are dangerous from the rest of society.
Civilized worlds are defined as self-sufficent worlds that can survive indefinitely in the event of communication collapse. These are worlds that have been totally colonized, where the race(s) on it has a strong foothold.
This is a world that is currently engaged in combat. Regardless of what classification was held before, if there is conflict on this planet, it is a combat world. For a planet to actually be listed as a combat world, however, typically the conflict must last 2+ months.
A colony world is a world that has only a few small areas of inhabited land. The people on the planet are not self-sufficent, still relying on shipments from the more developed worlds.
Industrial worlds are worlds handed over entirely to industry. The extraction of natural resources, the manufacturing of ships and goods, every part of the planet that can be exploited is exploited. Often these worlds are dead, as to prevent people protesting the extraction of resources.
A prison world is a place where prisoners are brought. While most planets do have a place for low-offense prisoners, drug offenders or thieves, prison worlds are for murderers, rapists, arsonists, people who cause physical harm to others through their actions.
However, in the DREL, the prison world is a very different place. It is a mix of industrial and prison world, where people who are deemed dangerous to the Republic or the people therein are made to work every day. Making mechas, tanks, shells, pots, pans, anything the Republic deems to be required. Some people who have escaped a prison world of the DREL gave a full report, which can be viewed here.
Though not literally planet-wide cities, the Ecumenpolises are covered in cities and arcologies, often numbering somewhere between 30 billion and 100 billion people. So far, there are only a handful of known Ecumenpoli, and they are largely the political, economic, and military centres of their regions, dominating hundreds of lightyears of space around them.