In this instructable I'll show you some basic concepts to understand the way Android storage works.
This will help you free precious space when the disk space notification comes up.
Step 1: System and User Points of View
From the system point of view your Android device storage is divided into several partions like:
/boot contains the kernel
/system contains the operating system
/recovery contains the recovery console
/data contains the user data
/cache contains frequent used data
and so on...
But from an user point of view what we see is that the storage space is divided in usually 3 spaces usually labelled something like phone, SD or mass storage and external SD.
Phone is where apps are installed along with other core data like phone book and sms.
This space is not directly accessible by the user and is usually the smallest.
Mass storage or SD is where applications store addtional data and where user files (like photos) are stored by default.
This space is accessible and its size varies a lot phone by phone.
External SD is the removable microSD card that we can plug in to expand our storage.
Now let's see what are the common problems and how we can solve them.
Step 2: Let's Free Some Space!
Now especially if you are using a middle-low end device and you install many apps or take many pictures your phone or mass storage will eventually get full and you won't be able to install applications or take pictures anymore.
What we can do is to plug in a external microSD card and move our files in a way that everything will continue working and we'll also have free space for new stuff.
This can be done moving our user files from mass storage to the external microSD using a file manager like ASTRO and setting our Camera app to save there by default and then moving some of our apps from phone into the mass storage.
This can't be done for all the apps and is done from the settings menu. There in the apps section you will find a list of all your apps and if you tap on one of them another screen will open in which there is a button "move to SD" that if is not greyed will perform the task.
In this way at the end of the process you'll have a phone storage with only the apps that can't be moved, a mass storage with the rest of the apps and an external SD with all you files optimizing the use of your device storage.
Step 3: Conclusion
Of course there are apps that automatize this process like App2SD and apps can also moved to the external SD. However in this way if you remove it from the phone the apps that are stored there will become unusable so my advice is to do it only if you still need to free more space.