A fitting is a weldable pipe fitting that allows for change of direction of flow, to branch off, reduce pipe size or attach auxiliary equipment. Pipe fitting is the occupation of installing or repairing piping or tubing systems that convey liquid, gas, and occasionally solid materials.

Common types of pipe fittings are:
Butt weld pipe fittings
Pipe flanges
Forged fittings
U bend tube
Fin tube
Forged fitting
Expansion joints
Abrasion resistant fittings
Accessories
  
Types of pipe fitting standards:
Some widely used pipe fitting standards are as follows:
ANSI: The American National Standards Institute
ANSI is a private, non-profit organization. Its main function is to administer and coordinate the U.S. voluntary standardization and conformity assessment system. It provides a forum for development of American national standards. ANSI assigns "schedule numbers". These numbers classify wall thicknesses for different pressure uses.

ASME: American Society for Mechanical Engineers
This is one of the reputed organizations in the world developing codes and standards. The schedule number for pipe fitting starts from ASME/ANSI B16.

The various classifications of ASME/ANSI B16 standards for different pipe fittings are as follows:
ASME/ANSI B16.1 - 1998 - Cast Iron Pipe Flanges and Flanged Fittings
ASME/ANSI B16.3 - 1998 - Malleable Iron Threaded Fittings
ASME/ANSI B16.4 - 1998 - Cast Iron Threaded Fittings
ASME/ANSI B16.5 - 1996 - Pipe Flanges and Flanged Fittings
ASME/ANSI B16.11 - 2001 - Forged Steel Fittings, Socket-Welding and Threaded
ASME/ANSI B16.14 - 1991 - Ferrous Pipe Plugs, Bushings and Locknuts with Pipe Threads
ASME/ANSI B16.15 - 1985 (R1994) - Cast Bronze Threaded Fittings
ASME/ANSI B16.25 - 1997 - Buttwelding Ends
ASME/ANSI B16.36 - 1996 - Orifice Flanges etc.
ASTM International: American Society for Testing and Materials

This is one of the largest voluntary standards development organizations in the world. It was originally known as the American Society for Testing and Materials (ASTM). This is a reputed scientific and technical organization that develops and publishes voluntary standards on the basis of materials, products, systems and services. This is a trusted name for standards. The standards covered by this organization covers various types of pipes, tubes and fittings, especially made of metal, for high-temperature service, ordinary use and special applications like fire protection. The ASTM standards are published in 16 sections consisting of 67 volumes.

AN: Here, "A" stands for Army and "N" stands for Navy
The AN standard was originally designed for the U.S. Military. Whenever, a pipe fitting is AN fittings, it means that the fittings are measured on the outside diameter of the fittings, that is, in 1/16 inch increments. For example, an AN 4 fitting means a fitting with an external diameter of approximately 4/16" or ¼". It is to be noted that approximation is important because AN external diameter is not a direct fit with an equivalent NPT thread.

BSP: British Standard Pipe
BSP is the U.K. standard for pipe fittings. This refers to a family of standard screw thread types for interconnecting and sealing pipe ends by mating an external (male) with an internal (female) thread. This has been adopted internationally. It is also known as British Standard Pipe Taper threads (BSPT )or British Standard Pipe Parallel (Straight) threads (BSPP ). While the BSPT achieves pressure tight joints by the threads alone, the BSPP requires a sealing ring.

DIN: Deutsches Institut für Normung
This refers to the industrial pipe, tube and fittings standards and specifications from the DIN, Deutsches Institut für Normung which in English means the German Institute for Standardization. DIN is the German national organization for standardization and is ISO member body for that country.

DIN standard designation
The designation of a DIN standard shows its origin where # symbolizes a number:

DIN # : Used for German standards having mainly domestic significance or designed as the primary step toward international status.
DIN EN # : Used for the German edition of European standards.
DIN ISO # : Used for the German edition of ISO standards.
DIN EN ISO # : Used if the standard has also been adopted as a European standard. 

Pipe fittings are used in various industries like:
Chemical / Petrochemical
Municipal
Food, Beverage, and Dairy
Oil and Gas
Power
Process Instrumentation
Pulp and Paper
Semiconductor
Marine & dredging
Sanitation
Flanges Used in Piping Systems
Irrigation
Residential
Road & highway construction
Ventilation etc.