The hobby of collecting consists of acquiring specific items based on a particular interest of the collector. These collections of things are often highly organized, carefully catalogued, and attractively displayed.
Since collecting depends on the interests of the individual collector, it may deal with almost any subject. The depth and breadth of the collection may also vary. Some collectors choose to focus on a specific subtopic within their area of general interest: for example, 19th Century postage stamps, milk bottle labels from Sussex, or Mongolian harnesses and tack. Others prefer to keep a more general collection, accumulating Star Trek merchandise, or stamps from all countries of the world.
Some collections are capable of being completed, at least to the extent of owning one sample of each possible item in the collection (e.g. a copy of every book by Agatha Christie). Collectors who specifically try to assemble complete collections in this way are sometimes called 'completists'. Upon completing a particular collection, they may stop collecting, expand the collection to include related items, or begin an entirely new collection.
The most popular fields in collecting have specialized commercial dealers that trade in the items being collected, as well as related accessories. Many of these dealers started as collectors themselves, then turned their hobby into a profession.
There are some limitations on collecting, such as financial constraints. One alternative to collecting physical objects is collecting experiences of a particular kind. Examples include collecting through observation or photography (especially popular for transportation, e.g. train spotting, aircraft spotting, metrophiles, and bus spotting), bird-watching, and systematically visiting continents, countries, states, national parks, counties etc.