Specialization is a feature that allows players to guide their city in a much more specific direction, as opposed to previous games where there was a "perfect balance" or often something close. It gives varied special buildings including processor factories, iron mines, gambling houses and more that let the player choose exactly what the city specializes in. The overall main specializations are mining of either coal or ore, oil drilling, a section which could be termed overall as "culture" which includes expo centres, landmarks, etc., creating special goods in factories ranging from alloys to TVs, trading and gambling.
Mining
Mining is relatively simple. A mine for the resource you want is built on top of the resources, and expanded as necessary. It will make deliveries to the local trade depot if it can, and the coal or ore can be used locally to boost industry, stockpiled, or sold. If used locally, it can be used to power local power stations. To expand it, you can build, with upgrades, places to refine the ore, smelt it, etc.
Drilling
Oil drilling is even simpler than mining. All that is done is the placing of oil wells which can be expanded, and they will deliver oil to the depot. Like coal, it can be used in some power plants, and it can also be refined into fuel.
Culture
Culture is a very broad term, and probably the closest you'll get to covering it overall. It refers to the expo building set, which lets you hold events in them which will earn revenue (if done properly; e.g. right time with decent number of people who will go) and the landmark set, which is a whole range of buildings you may build. Those buildings are all real landmarks, and they earn money from visitors as well as attracting tourists.
Gambling
Gambling is the seediest of the specializations. It involves the building of various casinos, starting off with a basic gambling house to much larger ones, which will have tourists come in and spend their money, providing income. However, it creates crime and undesirability in an area.
Factories
Factories is a section that probably arches into a couple others. Factories can be built to process alloys made from metal, which is gotten either by buying or mining and refining, and to process plastic (made from oil) plus the alloys into processors. The chain keeps going, with processors used in computers and televisions. In a way, factories are more of a category to shove all the leftovers that don't belong in trade.
Trade
Trade is in many ways a supplement to the other specializations than one itself. It refers to the buildings such as the trade depot and such which may be built and expanded for the storage, import and export of goods. It lets you build a trade HQ which benefits trade if you have done enough trading.
