Coins may be collected as objects in their own right for a number of reasons.
The scientific study of coins is a branch of numismatics.
Coin collectors usually select an area of interest and collect coins from that area. Popular interests include coins of a certain nation, ancient or medieval coins, world coins, and error coins. Often within these interests people choose even more specific specialties.
A collector of national coins often chooses coins of their own country. Popular ways to collect national coins include collecting one of every date and/or mint mark for a particular series (date/mint mark sets), and collecting a representative coin of each different series (type sets). For example, a date set in Great Britain may include one Queen Victoria large penny for each year, 1837-1901. In another example, a US type set might include an example of each varity of each denomination produced. Many collectors of national coins create unique combinations of date, mint mark, and type sets.
Collectors of ancient and medieval coins are usually more interested in historical significance than other collectors. Collectors of ancient and medieval might collect Roman, Greek, Celtic, Parthian, Merovignian, Ostrogothic, Jewish, or other coins. Specialties tend to vary greatly, but an example is collecting coins of a particular emperor.
Collectors of world coins are often interested in geography. They can travel the world vicariously through their collecting. A popular way to collect world coins is to collect representative examples from every country.
The collecting of error coins is a modern invention. Collectors of ancient and medievel coins shun coins with errors because ancient and/or medieval coins are unique (they didn't have mass-production back then). Collectors of modern coins seek collect them because they are so rare today. Types of coin errors include double strikes, off metal coins, off center coins, clipped coins, and mules (different denominations on two sides of one coin).
In coin collecting the condition of a coin is paramount; a high-quality example is often worth many times as much as a poor example. Collectors have created systems to describe the overall condition of coin. An older system describes a coin from poor to uncirculated. A newer system adds a 0-70 number grade, where 70 is a perfect coin. Several coin grading services will grade and "encapsulate" coins in an labeled, air-tight plastic holder. Two highly respected grading services are the Numismatic Guarantee Corporation (NGC) and Professional Coin Grading Service (PCGS). However, professional grading services are the subject of controversy because grading is subjective - a coin may receive a different grade by a different service, or even upon resubmission to the same service. Due to potentially large differences in value over slight differences in a coin's condition, some dealers will repeatedly resubmit a coin to a grading service in the hopes of a higher grade.