Adobe Flash or simply Flash, refers to both the Adobe Flash Player and to a multimedia authoring program used to create content for the Adobe Engagement Platform (such as web applications, games and movies).
The Flash Player, developed and distributed by Adobe Systems is a client application available in most dominant web browsers. It features support for vector and raster graphics, a scripting language called ActionScript and bidirectional streaming of audio and video.

Capabilities that require Flash :

Audio
As far as I know, no one has figured out a way to make an AJAX application play audio files. This can be important for the little details of an application. It can also be important if you’re making an immersive experience of some kind, as we were. We NEEDED sound.

Complex animation with arbitrary shapes
You’ll know if you need this or not. MindCanvas is a video-game like survey application, and we definitely needed vector graphics to make it come alive.

Sockets
Required for multiplayer games, stock-trading applications, etc). We had no need for this in our initial release, but we have a lot of ideas as to how to use MindCanvas to perform multi-player research exercises. We’d use flash communications server to do this in the flash world. There is no equivalent in the DHTML world.

Integration with client microphone / webca
We didn’t really need this at present (though it’s nice to know it’s there for the future). But many collaboration applications or consumer media sites will absolutely need this in the next few years.

Capabilities that Flash lacks
Rapid development: if your code needs to change quickly, or if it needs to be ready in a few weeks, Flash is a poor choice. In our case, the management UI for MindCanvas was originally built in Flash. When we had to change it to meet an early customer’s requirements, we realized that Flash really isn’t what you should be using to do rapid application modification.

Excellent handling of text
Something you take for granted on the web is that you will be able to display text of varying sizes in a way that is readable for the user. But font sizes that were easily “big enough” to be legible in html were fuzzy and hard-to-read in Flash. We bought several fonts from third-party vendors : these helped, but didn’t completely solve the problem.