Flash technology was formerly developed by Macromedia, but then in 2005 it merged with Adobe Systems. At present Adobe owns Flash technology. The latest version of Flash is version 10. The authoring tools for working with Flash developed too and Adobe Flash CS 5 Professional was released in April, 2010. There are also third party, mostly open source or shareware, applications. It should be noted that nowadays competition has become stronger, Microsoft Silverlight is gaining popularity and with wider and wider implementation of HTML 5 Flash may experience hard times.
We usually speak about file extension .FLA in the context of creating rich Internet applications or any Flash inclusions into a web page. FLA files are compiled into SWF format and then displayed to a web site visitor. But there often arises the necessity in the reverse process. It is possible to change Flash elements only if you have their source FLA files. In the majority of cases they are unavailable for various reasons. There exists specialized decompiling software which serves to extract FLA.
From the point of view of decompiling process we can answer the question above, "what is FLA", saying that FLA files are the source of graphical, audio, video elements on a web page which is to be obtained in order to edit SWF. It is quite convenient to use SWF to FLA Converter by Eltima to get the FLA files. The operation takes little time and the functionality allows even editing some elements on the fly.
To sum it up, file extension FLA is a widely spread format for web site construction and if it is necessary to apply any changes to an already created Flash element, FLA files should be extracted and edited.