The definition of the term, 'collection' is: a group of objects of one type that have been collected by one person or in one place.
The hobby of 'collecting' includes seeking, locating, acquiring, organising, cataloging, displaying, storing, and maintaining items that are of interest to an individual collector. Collections differ in a wide variety of respects, most obviously in the nature and scope of the objects contained, but also in purpose or presentation. The range of possible subjects for a collection is practically unlimited, and collectors have realised a vast number of these possibilities in practice, although some are much more popular than others.
Some collectors are generalists with very broad criteria for inclusion, while others focus on a subtopic within their area of interest. Some collectors accumulate arbitrarily many objects that meet the thematic and quality requirements of their collection, others—called completists or completionists—aim to acquire all items in a well-defined set that can in principle be completed, and others seek a limited number of items per category (e.g. one representative item per year of manufacture or place of purchase).
The most obvious way to categorise collections is by the type of objects collected. Most collections are of manufactured commercial items, but natural objects such as birds' eggs, butterflies, rocks, and seashells can also be the subject of a collection. For some collectors, the criterion for inclusion might not be the type of object but some incidental property such as the identity of its original owner––the possibilities are endless and we're utterly fascinated.