FIX:

The Financial Information eXchange (FIX) protocol is an electronic communications protocol developed for international real-time exchange of information related to the securities transactions and markets.
FIX is widely used by both the buy side (institutions) as well as the sell side (brokers/dealers) of the financial markets. FIX is the standard for front office messaging. FIX is the standard for front office messaging.
There are two main groups of messages - admin and application. The admin messages handle the basics of a FIX session. They allow for a session to be started and terminated and for recovery of missed messages. The application messages deal with the sending and receiving of trade-related information such as an order request or information on the current state and subsequent execution of that order.

FIX Message Layout:
The message fields are delimited using the ASCII 01 character. They're composed of a header, a body and a trailer. The header must contain the 8 (BeginString), 9 (BodyLength), 35(MsgType) tags. The trailer is always terminated by a 10 tag, a checksum.
Header+Body+Trailer : FIX Content
Example of a FIX message : Execution Report
8=FIX.4.2 | 9=67 | 35=8 | 49=PHLX | 56=PERS | 11=ATOMNOCCC9990900 | 52=20071123-05:30:00.000 | 20=3 | 150=E | 39=E | 55=MSFT | 167=CS | 54=1 | 38=15 | 40=2 | 44=15 | 58=PHLX EQUITY TESTING | 59=0 | 47=C | 32=0 | 31=0 | 151=15 | 14=0 | 6=0 | 10=102 |