Your created wealth is the sum product of all the labors of your fortress. It is visible on the status screen once you have a broker with the appraisal skill and a bookkeeper maintaining your records, and will update with the continual maintenance of your stocks by the bookkeeper.
Wealth is the sum of everything of value in your fortress: basically, everything except corpses and remains, which have absolutely no worth whatsoever. This includes stones, buildings, engravings, and any and every kind of created good, all of which contribute to your total wealth to various degrees. Artifacts are usually one of the largest influences on fortress wealth. Artifacts made of precious resources and heavily decorated can easily be worth thrice the value of the rest of your fortress in the early years.
The display of your wealth on the main status screen is broken up into several categories: weapons, armor and garb, furniture, other objects (like finished goods), architecture (buildings and such), displayed, and held/worn (items created internally that have been claimed by dwarves, like clothing).
 Imports and exports
Items made off-site are not counted in your total fortress wealth, and are instead listed as imports. This only applies so long as the object is unchanged; a decorated imported object will be made your own, and its value will be moved from imports to wealth. This is important when trading with caravans, as they will not accept goods stolen or lost by a previous caravan of that civilization. It is listed under "Imported Wealth".
Similarly, items made in the fortress that leave the map on a caravan, be it dwarven, elven, human or goblin, are counted as exports, listed under "Exported Wealth." 5k in offerings (not trades) to dwarven caravans is one of the prerequisites for the monarch to arrive.
Wealth influences various game features, some of them good, some of them bad.
On the plus side, it increases the amount of migrants you get per wave. Assuming that you are equipped to handle the new dwarves, this is usually a good thing, except when you are producing so much wealth that your migrant arrivals outstrip your ability to house and feed them or give them useful things to do. Drowning in migrants is a very real danger; make sure you are always equipped with surplus beds and food.
A certain level of exports and overall wealth is required to go up the noble ladder: a baron requires 100k of wealth, a count 200k, and a duke 300k, while the monarch has some fairly complicated requirements based on a few different categories.
On the negative side, more wealth attracts more attacks. At first, this will be an above-average amount of thieves, but as the game progresses and your wealth continues to grow, this will develop into ambushes, sieges, and visits from megabeasts, all of which are attracted to increasing amounts of wealth. This keeps the game from being boring, but too much fun is also a bad thing; if you have a hard time dealing with the numerous waves of immigrants, you're probably not equipped to deal with a full-on siege.
 Building and limiting wealth
Building wealth is simple - just commit more people to useful industries and continue growing. You will want to establish a major industry and commit a dwarf to it (producing finished goods is the easiest way), allowing you to spend grand sums on caravans and get everything you absolutely need quickly and painlessly. If you intend to go this route, be sure to create tons of extra bins to ease the transfer of items to the trade depot and prevent stockpiles from becoming too full.
On the opposite side, there are the fortresses that would much rather establish a baseline of sorts before embarking on an expansionist binge - getting a full defensive grid up, for instance, or penetrating the aquifer without having to waste precious reserves on more dwarves and more enemies. These players concentrate on low-value activities like carpentry, masonry, and mining, and only produce enough trade goods to get what is necessary from the caravans. Although they are slower to grow, they also afford their players more time to plan and to lay the groundwork for the future of the fortress. Note that, no matter what, you will have to deal with some growth - besides the natural expansion of your fortress, there's also the issue of artifacts. Do what you will, but every once in a while a dwarf will claim a workshop and produce a valuable trinket, and all you can hope for is that it's not worth too much.
Most players choose to walk the middle line, getting together the necessary industries but concentrating on the metal industry early on to get together their arms and armor. Although steel is worth its weight in gold, it is much more useful in deflecting goblin arrows from your fortress' defenders than it would be as a statue looking pretty in your lobby.