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There are three types of potential memory on a BB: 1) Application Memory, 2) Device Memory, and 3) Media/SD Card Memory
Application Memory -- This is the most crucial; it is the protected (not user accessible), dedicated, and fixed (in size) space that is available as the destination for the installation of applications (plus some application storage, overhead and such). You cannot touch AppMemory. You cannot improve the maximum AppMemory that your BB has. It is what it is. Applications can only install here...there is no option.
Device Memory -- This is space on your BB that you can touch to store files, pictures, media, etc. Typically, it is not terribly large, but it is available to you.
SD/Media Card Memory -- This is what it says...your SD card, for you to store files, media, pictures, etc. It can be as large as your BB OS can support...see this KB: http://www.blackberry.com/btsc/KB05461
On some devices/OS levels, Device Memory and Application Memory are combined into "File Free"...these are, IMO, a little harder to administer as you really can't see well just how much AppMemory you have. Nevertheless, here are some guidelines to use:
Lastly, it is always important to properly close applications when you are done with them. Using the Back or the Red key will not do this -- those leave it to the app to decide what to do...and some will leave themselves resident in memory, consuming resources on your BB, slowing the overall performance. Rather, to close an app, press the BBKey and select "Close" or "Exit"...that will force the application to be closed, freeing up for your new use the resources it was consuming. Some apps will always remain running (typically -- BBMessenger, Browser, Homescreen, Phone, and Messages)...but, you should still close them properly - especially the Browser...if it is left on an active web page, it will not only consume extra resources but battery power as well.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.