A few themes are common and are often combined into a goal for a collection.
Many collectors attempt to obtain a sample from every country which has issued a coin. In contrast to those who collect coins from all countries, many collect coins from only one country. The country selected is often their own.
Rather than collecting one example of a type, some collectors prefer to collect by year. For example, they might collect one Lincoln cent for every year from 1909 to the present. This is probably one of the most practical ways to collect US currency. Most bookstores sell specially designed books, or coin albums, for the purpose of collecting coins by year.
Many collectors consider that different mint marks give sufficient differentiation to justify separate representation in their collection. This increases the number of examples needed to complete a collection from one per year to several per year. Some mintmarks are more rare than others. This is a great way to collect coins and to have a great time doing it.
As mints issues many thousands or millions of any given coin, there are generally multiple sets of dies used. Occasionally these dies will be slightly different. Generally this is in a very small detail, such as the number of leaves on the ear of corn on the recent US Wisconsin state quarter. Varieties are more common on older coins, when the coin dies were hand carved.
Often a collection consists of an example of major design variants for a period of time in one country.
Collectors with an interest in a subject (e.g. ships or dogs) may collect only coins depicting that interest.
For some, the composition of the coin itself is interesting. For example there are several collectors of only bimetallic coins. Normally only precious metals like gold, silver, copper and platinum fit this category.
Many collectors restrict themselves to coins issued after the 18th or 19th century, while others collect ancient and medieval coins. Coins of Roman, Byzantine, Greek, Indian, Celtic, Parthian, Merovingian, Ostrogothic, and ancient Israelite origin are amongst the more popular ancient coins collected. This theme also includes Mughal period, British Period, Republic Indian Peroid and coins issued during kingdom of various kings in India like Cholas, Maurayas, Guptas, Vijaynagars kingdoms. Specialties tend to vary greatly, but some approaches include the collection of coins minted during a particular emperor's reign or a representative coin from each emperor. Some collectors also like to collect all coins issued during the administration of a particular individual like RBI governor, finance secretory, prime minister, treasurer. For example, in India RBI governor issued new paper money with many added features while existing one also circulates. Coins are often a reflection of the events of the time in which they are produced, so coins issued during historically important periods are especially interesting to collectors and falls in this category.
Collectors with an interest in acquiring signatures of Finance Secretory (in India), RBI governor (in India), Chairman of Board of Commissioners of Currency (in Singapore), Gabenor (in Malaysia), treasurer (in United States and other countries) on the paper money (e.g. presently RBI governor Dr. D. Subbarao signs on all Indian Paper Money (except on Re.1), pastly Finance Secretory signs on Re. 1 Paper Money)
Often a collection consists of an example of number series for a period of time in one country. This will be encouraged as the number would be lucky for collector or represents some mythological Symbols (e.g.786 for Muslim religion, they store that currency bearing 786 on them though they may not fall in any of the below mentioned category of "Types of Collector"). Some also interested in collecting paper money bearing a numerical series, an A.P., a G.P., an H.P., an number of identical numbers (90505050,99910299),others (1030507,987654,24680,1235813,1491625).
Printed Value collections
Often a collection consists of currency of a specific Printed Value. For example of 1 (includes 1 Indian Rupee, 1 American Dollor, 1 Singapore Dollor etc.)
Collectors with an interest in acquiring large volumes of a particular coins (e.g. as many pennies as they can store). These usually are not high-value coins, but the interest is in collecting a large volume of them either for the sake of the challenge, as a store of value, or in the hope that the intrinsic metal value will increase.
by : wikipedia.org