What is HL7

HL7 is a standard for health’s clinical and administrative data. One of objective of this standard is to develop a coherent, extendible standard that permit structured, encoded health care information of the type required to support patient care, to be exchanged between computer applications while preserving meaning.

How a HL7 Message looks like

MSH|^~\&|GHH LAB|ELAB-3|GHH OE|BLDG4|200202150930||ORU^R01|CNTRL-3456|P|2.4
PID|||555-44-4444||EVERYWOMAN^EVE^E^^^^L|JONES|19620320|F|||153 FERNWOOD DR.^^STATESVILLE^OH^35292||(206)3345232|(206)752-121||||AC555444444||67-A4335^OH^20030520
OBR|1|845439^GHH OE|1045813^GHH LAB|15545^GLUCOSE|||200202150730|||||||||555-55-5555^PRIMARY^PATRICIA P^^^^MD^^|||||||||F||||||444-44-4444^HIPPOCRATES^HOWARD H^^^^MD
OBX|1|SN|1554-5^GLUCOSE^POST 12H CFST:MCNC:PT:SER/PLAS:QN||^182|mg/dl|70_105|H|||F
Yes… Looks strange at first sight… but believe me, it is perfectly logic.

Dissecting the message
An HL7 message consists of the following data elements.

Message type

An HL7 message type is a unique identifier for the business purpose of a message. Every message must contain a message type id as way to announce the purpose of the message. For example, ADT is a unique message ID to Patient Administration.
However, it is rather not a unique classification on the structure of a message. One message type can have more than one message structure.
The message type is advertised in the message header segment

Message event

The message event, sometimes called a trigger, is a unique identifier to the context in which message is generated. The message event consists of an upper case letter and two digits. For example, A01 is for admission/visit notification and A61 is for changing consulting doctor. Both A01 and A61 are used with ADT messages
Event type is advertised in the message header segment.

Message structure

The message structure is a data structure used to express an association of a message type with an event for a class of messages. Each message structure also contains a unique ID.
It structurally consists of a well-defined list of HL7 segments. Segments can be optional, and can repeat. There is no limit on how many times a segment can repeat.
Segments can be aggregated together to form a segment group, which can repeat as well. In the standard specification, segment group is indicated by {} or [], where {} signifies repetition and [] signifies optionality.
Relative position of segments in a message structure and segment groups is well defined. At the message structure level, segment is the atomic data type.