The hobby of collecting is a wide-ranging sphere of activity which essentially involves the acquisition of specific items based on a particular interest of the collector. Collections are often highly organised, carefully catalogued and attractively displayed.
Since collecting depends on the interests of the individual collector, its scope can vary from one to several subjects and, equally, the depth and breadth of a given collection may also vary. Some collectors choose to focus on a specific sub-topic within an area of general interest: for example, a philatelist may specialise in postage stamps of the 19th century. Others prefer to build a more generic collection, such as acquiring stamps from all the 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 – for example, a copy of every book by Agatha Christie. Collectors who seek the acquisition of complete collections in this way are sometimes called "completists". Upon completing a particular collection, they may complement it by the inclusion of related items, or begin an entirely new collection.
The most popular fields in collecting have specialised commercial dealers who trade in the items being collected, as well as related accessories. This is often the case with dealers in stamps and coins. Many of these dealers started as collectors themselves before turning their hobby into a profession.
There are some constraints on collecting, such as available space and limited finance. An alternative to the collection of physical objects is the "wishlist" or "bucket list" syndrome whereby the person accumulates experiences. Examples include recorded observation or photography of particular phenomena such as transport – e.g., train spotting, aircraft spotting and bus spotting. Ornithology is potentially another example if the enthusiast attempts sightings of as many birds as possible. There are football fans who seek to visit as many football grounds as possible. In a similar vein, other travellers systematically visit countries, British counties, American states, national parks, etc.