Tip: Windows Mobile — How Much Room Do I Have?
"Storage" refers to the amount of room you have to download new applications or add new user files such as photos and documents. The "Program" numbers refer to the amount of your system's resources currently in use by active applications.
[Some nitty-gritty for the techie and/or uber-curious: Windows Mobile 5 introduced, and Windows Mobile 6 continued, a persistent memory storage system that keeps one part of your device's memory allotted specifically for running applications (the Random Access Memory), while all your operating system, application and user files live in a segregated area (of Read Only Memory). For more, see here.]
Most devices offer you the added storage of a microSD card, which you can use for applications or data. To see how much room remains on your card, click the Storage Card tab from the same Memory screen.
If the reason you're concerned about memory is that your device is working slowly, you may need to close some programs that are running in the background. Here's what you do:
From Settings-->System-->Memory press the Running Programs tab. From here you can close applications that you aren't using. Windows Mobile often keeps a program running in the background after you click the X to close it. This is where you can actually exit the software, freeing up system resources for other applications you now want to use.