Money, goods, gold, property, all form wealth. It is the absolute backbone of an empire's economy.
Methods to Create Wealth
With money being so important to your economy, the obvious question is how does one get it? There are several methods, which leads naturally to the questions of which is necessary, which is best, and so on. The answer depends on your play style to some extent, but as to which are useful the answer is "all of the below". Not only just to use them but to try to maximize one's return. As just one example, on a large map it can be very handy to build additional survey ships (either armed or built with an escort) to scavenge anomalies for extra cash, among other things. How much emphasis and effort should be put into each method is a question of experience. The methods are listed in a rough order of importance; but this is always a matter of situation (especially the galaxy makeup) and player style. Here are some general comments about each type as details should be found separately in other areas of the Wiki.
Colony Production
For most of the game the wealth generated by colonies will be a large part of one's income. Financial buildings (starting with the market or similar building) are used to raise the gross income of a colony. It might be suggested that the starting commerce technology for most factions, Xeno commerce, needs to be researched fairly early so that markets can be started; otherwise lack of funds may prove a serious problem for an empire. Specialization is important here where possible so that as little of colony production needs to be dedicated to money as possible. Planning ahead can be useful; for example, the financial capital provides a huge increase in income and has high adjacency bonuses. Thus a player should be on the lookout for an appropriate colony to set up with the best, centralized tile reserved for the financial capital. By such means it should be possible to run only a very few "wealth" planets and maintain a balanced economy, while other colonies concentrate on research, manufacturing, or perhaps influence.
Tourism
Tourism provides a safe, steady income that grows as your empire grows.
Surveys
The player normally starts with one survey ship that can study Anomalies; the player can build additional survey ships once he/she has researched the Survey technology. Placing a survey module on a ship will provide the survey capability. Note that the player's original survey ship is armed and at full health can deal easily with pirates that may be guarding Anomalies. Survey ships built later will need armaments or be escorted by combat vessels. Anomalies frequently supply cash rewards; these are rather hit-or-miss, as you may get 1 BC or perhaps 500 BC. But uncovering a number of anomalies should produce a valuable addition to one's income stream, particularly early in the game when the temptation to spend BC to hurry things can be compelling.
Relics
Relics can be used by deploying a constructor within range of the relic, and choosing a precursor module. The first such module is the Xeno Archeology Lab which gives a 10% empire-wide boost depending on the type of relic; higher level modules can be added after researching the appropriate technologies. Being able to grab a wealth relic is of course situation dependent, but like all other relics it makes sense to make a strong effort to secure one. In fact a war over relic(s) is not out of the question. Even at the first level, a 10% increase for your entire empire is a big advantage.
Trade
Trade requires an empire to create a trade route. To do so requires sending a freighter built in the shipyard to another faction's colony; hence, the freighter must have sufficient range to reach the target colony. To complete a new trade route requires that one's empire has the capacity for an additional route. Most factions start with the capability for one route, and can increase this by researching the appropriate technologies. There is also a UP resolution that if passed gives every faction two addition trade routes. The return from a trade route, considering the time building and sending a freighter will not seem large at first, but the longer a trade route exists the more lucrative it will become. There is a pragmatic ideology trait that allows luxury trade routes that further enhance the income.
Of particular importance is that the existence of a trade route improves the diplomatic relations between the connected factions. This may in some situations be more important than the income, when for example one is fighting a war on one flank and is hoping to keep other factions at least neutral for the duration.
Sale of Items to Other Factions
One can consider trading Technology, Ships, Strategic Resources, Trade Resources, Colonies, and Treaties to other factions for BC. Of course some deals are going to be far more attractive than others, but in a particular situation any of these items might be sensible to trade. For example, at times one might obtain a colony that becomes untenable due to distance and enemy influence. Selling the colony to another faction then might be a smart move. A typical early game treaty sale to another faction is Open Borders, which allows ships from either side to move into each other's zone of influence without a diplomatic penalty. In any case of a sale of any item to the AI, a careful judgement needs to made to decide whether it is appropriate, and whether the BC (or other items) received is sufficient compensation.
Trade Resource
Some trade resources affect wealth, for example, Ultra Spice increases gross income. Techapod Hive reduces maintenance.
Decommissioning
When a ship becomes obsolete it should be considered whether it should be decommissioned. This will return some BC and it will reduce maintenance. This should be considered carefully as even an old ship may have some useful value either to add to the defense of an important location or just to act as a picket along one's borders, so that enemy ships cannot penetrate unseen.
Economizing
A critical part of creating wealth is to minimize spending. For example, just about everything you build requires maintenance. Hence a colony producing 100% manufacturing for ship building has no use for a research building, even if that building is sitting on a bonus tile. Clearly it should be destroyed to save the maintenance (and some useful building substituted). In essence, decommissioning ships that are out of date is also reducing maintenance, as mentioned above. On a larger scale, judging the size of fleets required for a given task is an important skill to learn so that one does not "overbuild" and spend maintenance for impressive fleets that actually are not being used.