Domains are an expression of a god’s expertise and influence on the world. A god with fewer domains is most likely less powerful and has fewer followers than a god with many domains, by sheer amount of time spent in the world. Every god has at least one domain.
Domains come in multiple levels, each with varied degrees of specialization. The cheapest domains to acquire, lesser domains, are the most specialized and likely most commonly acquired. Any god may claim any Greater, Moderate, or Lesser domain regardless of if it has already been claimed by another god. However, each domain may only be claimed once as the starting domain of a god, and new gods may start with domains that have already been claimed (unless another god already started with that domain)
A god can share any domain he posses with another god at any time he wishes. However, the domain can be reclaimed at anytime by the original owner.
While Greater domains tend to be wider in scope, Lesser domains exert greater control over the limited element. A god of Dragons (a Moderate domain) would have more control over dragons than a god of Life (a Greater domain), but a god of Chromatic Dragons (a Lesser domain) would have greater control over Chromatic Dragons than the god of Dragons, and a god of Red Dragons (also a Lesser Domain) would have greater control over those creatures than any other mentioned god.
Greater Domains are defined as Primary Natural Forces, Fundamental Concepts, and Alignments.
Moderate Domains are defined as Secondary Natural Forces, Knowledge Categories, Creature Categories, Abilities, and Mortal-related Concepts.
Lesser Domains are defined as Tertiary Natural Forces, Specific Concepts and Actions, Specific Creature Groups, Ability-based concepts, Skills, Damage Types, and Specific Locations.
A list of Example domains of all three levels, for reference. These are by no means the only domains available. Those Domains that are wiki-linked have been already claimed; however, more than one god can claim Greater, Moderate, and Lesser domains.
Aether, Air, Balance, Chaos, Change, Competition, Creation, Darkness, Death, Desire, Destruction, Discipline, Earth, Evil, Fate, Fire, Good, Harmony, Ice, Knowledge, Law, Life, Light, Magic, Nature, Order, Peace, Secrets, Spirit, Time, Unity, Void, War, Water
Aberrations, After-life, Agriculture, Animals, Bravery, Charisma, Civilization, Commerce, Conquest, Corruption, Decay, Destiny, Diseases, Dragons, Dreams, Evolution, Fertility, Fey, Glory, Healing, Honor, Invention, Justice, Language, Liberation, Luck, Madness, Manipulation, Mercy, Metal, Metalworking, Moon, Murder, Music, Necromancy, Ocean, Passion, Pestilence, Plant, Protection, Psionics, Purification, Rebirth, Renewal, Retribution, River, Sky, Slaughter, Stars, Storm, Strategy, Strength, Sun, Travel, Trickery, Truth, Undead, Underground, Utility Magic, Zeal
Athletics, Blacksmiths, Blue Dragons, Cavern, Chromatic Dragons, City, Courage, Day, Divination, Dwarf, Elf, Endurance, Envy, Evil Dragons, Farmers, Fear, Fighters, Force, Forest, Fungus, Gold Dragons, Greed, Halfling, Harvest, Hatred, History, Illusion, Insight, Joy, Kobold, Lies, Lightning, Love, Lust, Machinery, Meditation, Mountains, Night, Nobility, Oracle, Pain, Planning, Pleasure, Poison, Pride, Revelry, Rogues, Rune, Scrying, Sea, Sight, Sleep, Song, Spider, Stealth, Suffering, Tactics, Teleportation, Trade, Traps, Trees, Tyrants, Vanity, Victory, Wind, Wizards, Wrath