MARC is the great-granddaddy of data exchange formats, based on ANSI standard Z39.2, American National Standard for Bibliographic Information Exchange and ISO 2709: 1996, Information and Documentation - Format for Information Exchange. MARC was first rolled out publically by OCLC in 1967. Since then, it has become the backbone for metadata exchange among libraries and archives around the world. At first there were various, flavours of MARC - however after a decade of negotiation and standards alignment, in 1977 UNIMARC was developed to allow conversion between various regional formats. Then, after many years of descreptive cooperation, the American USMARC & Canadian CAN/MARC standards were integrated into MARC 21 (detailed below) in 1999. This finally allowed easy standardized sharing and exchange of cataloguing data for libraries and institutions throughout North America. Negotiations are underway to bring UKMARC under this umbrella in the near future.

MARC actually contains five data formats: bibliographic, authority, holdings, community information and data classification - the last four are, broadly speaking, technical elements used internally by libraries, archives and other institutions to track classification, usage and ensure record integrity. The Bibliographic format however, makes up what the public sees. As you can see below, the records would be expansive if all the fields & text were used. Instead, as Sylvar outlines above, only the three digit tags applicable to a record are used. Each field then has specifically defined subfields for the description of each data element.

For example, the Publication field is tag 260, and has three subfields: a) place of publication, b) name of publisher and c) date of publication. Each subfield is within a tag is marked off by a delimiter (usually $, # or _ characters) so the catalogue software knows where the data for each field starts and ends. Hence, the 260 ## $aBoston :$bLittle Brown,$c1989. A bit confusing at first, once you know the major fields and subfields, it’s not so hard to see how it all fits.

So. Need to get your data organized, regardless of format? Here's how the descriptive/bibliographic section breaks down:
00X Control fields - General information
001 Control number
003 Control number identifier
005 Date and time of latest transaction
006 Linking field
007 Physical description fixed field
008 Fixed fields
01X-09X Variable data fields - General information
010 Library of Congress card number
015 National bibliography no.
016 NLC bibliographic record control number**
017 Copyright registration no.
018 Copyright article-fee code
019 Superseded OCLC control no.*
020 International standard book number
022 International standard serial number
024 Other standard identifier
025 Overseas acquisition no.
027 Standard technical report number
028 Publisher number for music
030 Code designation
032 Postal registration number
033 Capture date and place
034 Mathematical data codes
035 System control number
036 Original study number for computer data files
037 Stock number
039 Level of bibliographic control & coding detail
040 Cataloguing source name
041 Language codes
043 Geographic area code
044 Country of publishing/producing entity code
045 Time period of content
047 Form of musical composition code
048 Number of instruments or voices
050 Library of Congress call number
051 LC copy, issue, offprint statement
052 Geographic classification code
055 Call numbers / class numbers assigned in Canada
060 NLM call number
061 NLM copy statement
066 Character sets present
070 NAL call number
071 NAL copy statement
072 NAL subject category number
074 GPO item number
080 Universal decimal number
082 Dewey decimal classification number
086 Government document number
088 Report number
09X Local call numbers
090 Local call number and holdings data (Historical)**
098 On order information (Historical)**
1XX Main entries - General information
100 Main entry - Personal name
110 Main entry - Corporate name
111 Main entry - Meetings and conferences
130 Main entry - uniform title
20X-24X Title and title-related fields - General information
210 Abbreviated key title
222 Key title
240 uniform title
241 Romanized title
242 Translation of title by cataloguing agency
243 Collective uniform title
245 Title statement
246 Varying form of title
247 Former title or title variation
250-28X Edition, imprint, etc. fields - General information
250 Edition statement
254 Musical presentation statement
255 Cartographic mathematical data
256 Computer file characteristics
257 Country of producing entity for archival films
260 Publication, Distribution, etc. (Imprint/release)
263 Projected publication date
270 Primary address
3XX Physical description, etc. fields - General information
300 Physical description
306 Playing time for sound recordings
307 Hours, etc.
310 Current publication frequency
321 Former publication frequency
340 Physical medium
342 Geospatial reference data
343 Planar coordinate data
351 Organization and arrangement of materials
352 Digital graphic representation
355 Security classification control
357 Originator dissemination control
362 Dates of publication and volume designation
4XX Series statements - General information
400 Series statement/Added entry - personal name/title
410 Series statement/Added entry - corporate name/title
411 Series statement/Added entry - conference or meeting/title
440 Series statement - title
490 Series statement
5XX Notes - General information
500 General note
501 "With" note
502 Dissertation note
504 Bibliography/discography note
505 Contents note (Formatted)
506 Restrictions on access note
507 Scale note for graphic material
508 Creation/production credits note
510 Citation/References note
511 Participant or performer note
513 Type of report and period covered note
514 Data quality note
515 Numbering peculiarities note
516 Type of computer file or data note
518 Date/time and place of an event note
520 Summary, etc. note
521 Target audience note
522 Geographic coverage note
524 Preferred Citation of described materials
525 Supplement note
530 Additional physical form available note
533 Reproduction note
534 Original Version note
535 Location of originals/duplicates note
536 Funding information note
538 System details note
540 Terms governing use and reproduction note
541 Immediate source of acquisition note
544 Location of associated archival materials note
545 Biographical or historical note
546 Language note
547 Former title complexity note
550 Issuing body note
551 Entity and attribute information note
555 Cumulative index /finding aids note
556 Information about document note
561 Provenance note
562 Copy and Version identification note
565 Case file characteristics note
567 Methodology note
580 Linking entry complexity note
581 Publications about described materials note
583 Action note
584 Accumulation and frequency of use note
585 Exhibitions note
586 Awards note
59X Local notes
6XX Subject access fields - General information
600 Subject Added entry - personal name
610 Subject Added entry - corporate name
611 Subject Added entry - meeting name
630 Subject Added entry - uniform title
650 Subject Added entry - topical term
651 Subject Added entry - geographic name
653 Index term - uncontrolled
654 Subject Added entry - faceted topical terms
655 Index term - genre/form
656 Index term - occupation
657 Index term - function
658 Index term - curriculum objective
680 PRECIS descriptor string**
69X Local subject access fields
700-75X Added entries - General information
700 Added entry - personal name
710 Added entry - corporate name
711 Added entry - meeting name
730 Added entry - uniform title
740 Added entry - uncontrolled related/analytical title
752 Added entry - hierarchical place name
753 System details access to computer files
754 Added entry - taxonomic identification
755 Added entry - physical characteristics
76X-78X Linking entries - General information
760 Main Series entry
762 SubSeries entry
765 Original language entry
767 Translation entry
770 Supplement/special issue entry
772 Parent record entry
773 Host item entry
775 Other edition entry
776 Additional physical form entry
777 "Issued with" entry
780 Preceding entry
785 Succeeding entry
786 Data source entry
787 Non-specific relationship entry
800-840 Series added entries - General information
800 Series Added entry - personal name
810 Series Added entry - corporate name
811 Series Added entry - meeting name
830 Series Added entry - uniform title
841-88X Holdings, alternate graphics, etc. - General information
850 Holding institution
852 Location
856 Electronic location and access
880 Alternate graphic representation
886 Foreign MARC information field