The Dewey Decimal Classification (DDC) system is a general knowledge organization tool used in libraries and other institution across the world.
In the DDC, basic classes are organized by disciplines or fields of study. At the broadest level, the DDC is divided into ten main classes, which together cover the entire world of knowledge. Each main class is further divided into ten divisions, and each division into ten sections (not all the numbers for the divisions and sections have been used).
From the Dewey FAQ: http://staff.oclc.org/~dewey/dewey.htm
Dewey: The 000s (Computer science, information & general works)
000- Computer science, knowledge & systems
010- Bibliographies
020- Library & information sciences
030- Encyclopedias & books of facts
040- [Unassigned]
050- Magazines, journals & serials
060- Associations, organizations & museums
070- News media, journalism & publishing
080- Quotations
090- Manuscripts & rare books
The 100s (Philosophy and psychology)
100- Philosophy
110- Metaphysics
120- Epistemology
130- Parapsychology & occultism
140- Philosophical schools of thought
150- Psychology
160- Logic
170- Ethics
180- Ancient, medieval & eastern philosophy
190- Modern western philosophy
Dewey: The 200s (Religion)
200- Religion
210- Philosophy & theory of religion
220- The Bible
230- Christianity & Christian theology
240- Christian practice & observance
250- Christian pastoral practice & religious orders
260- Christian organization, social work & worship
270- History of Christianity
280- Christian denominations
290- Other religions
Dewey: The 300s (Social sciences)
300 Social sciences, sociology & anthropology
310 Statistics
320 Political science
330 Economics
340 Law
350 Public administration & military science
360 Social problems & social services
370 Education
380 Commerce, communications & transportation
390 Customs, etiquette & folklore
Dewey: The 400s (Language)
400 Language
410 Linguistics
420 English & Old English languages
430 German & related languages
440 French & related languages
450 Italian, Romanian & related languages
460 Spanish & Portuguese languages
470 Latin & Italic languages
480 Classical & modern Greek languages
490 Other languages
Dewey: The 500s (Science)
500 Science
510 Mathematics
520 Astronomy
530 Physics
540 Chemistry
550 Earth sciences & geology
560 Fossils & prehistoric life
570 Life sciences; biology
580 Plants (Botany)
590 Animals (Zoology)
Dewey: The 600s (Technology)
600 Technology
610 Medicine & health
620 Engineering
630 Agriculture
640 Home & family management
650 Management & public relations
660 Chemical engineering
670 Manufacturing
680 Manufacture for specific uses
690 Building & construction
Dewey: The 700s (Arts & recreation)
700 Arts
710 Landscaping & area planning
720 Architecture
730 Sculpture, ceramics & metalwork
740 Drawing & decorative arts
750 Painting
760 Graphic arts
770 Photography & computer art
780 Music
790 Sports, games & entertainment
Dewey: The 800s (Literature)
800 Literature, rhetoric & criticism
810 American literature in English
820 English & Old English literatures
830 German & related literatures
840 French & related literatures
850 Italian, Romanian & related literatures
860 Spanish & Portuguese literatures
870 Latin & Italic literatures
880 Classical & modern Greek literatures
890 Other literatures
Dewey: The 900s (History & geography)
900 History
910 Geography & travel
920 Biography & genealogy
930 History of ancient world (to ca. 499)
940 History of Europe
950 History of Asia
960 History of Africa
970 History of North America
980 History of South America
990 History of other areas