The two main library classification systems are the Dewey Decimal system and the Library of Congress system. Both are expandable tools for sorting books into categories, but they differ in how they categorize books and in the sorts of libraries that use them. They also differ in their basic organizational scheme.
Basics of the Library of Congress System
The Library of Congress system has 21 broad categories, each represented by one letter of the alphabet. For example, books on philosophy, psychology and religion all are have codes starting with the letter B. Books on education start with the letter L. History of the Americas gets two letters: E and F. Many classes have subclasses identified by a second letter; for example, books on the history of education start with LA. Other classes begin with a single letter and then numbers (classes E and F, for example). Subsequent numbers further divide the categories.
Basics of the Dewey Decimal System
The Dewey Decimal System first classifies books into nine broad categories, each identified by a number in the hundreds. For example, books on religion start are in the 200s; books on social sciences are in the 300s. Numbers are assigned within those categories; for example, 201 is for books on the philosophy of Christianity. Some topics use the 10s column for subdivision; for example, books on law are in the 340s; books on criminal law start with 345. Further division of categories is done by adding decimal numbers.
Differences Between the Two Systems: Outline Versus Hierarchy
The Library of Congress system is an outline, much like one you would use to organize an essay. Subtopics are dealt with by indentation. The Dewey Decimal system is a hierarchy: Subtopics are dealt with by using hundreds, then tens, then units and then decimal places.
Differences Between the Two Systems: Which Libraries Use Them
The Dewey Decimal System is used by more libraries, but the Library of Congress system is used by some of the biggest libraries -- most obviously the Library of Congress itself. Many universities use the Library of Congress system, but public libraries mostly use the Dewey Decimal System.