If You're Running Ice Cream Sandwich (4.0) and Above
If you have a shiny new phone with Ice Cream
Sandwich or above, screenshots are built right
into your phone! Just press the Volume Down
and Power buttons at the same time, hold them for a second, and your phone will take a
screenshot. It'll show up in your Gallery app for you to share with whomever you wish!
If You're Running Android 2.3 and Below
Unfortunately, Android 2.3 and below doesn't
have screenshotting built-in. However, some
devices (like many Samsung phones) do have
these features, but it varies from phone to
phone. for example, on many Samsung phones, you can press the Home and Power buttons at the same time to take a screenshot. Google around to see if your own phone has a built-in shortcut.
If your phone doesn't have a shortcut built-in,
you'll need to use an app like No Root Screenshot It . You'll need to install it on your phone and install its free companion app on your computer, which will enable screenshots. You'll have to plug your phone back into your computer every time you reboot it, but you'll be able to take screenshots directly from your phone which is great.
If You're Rooted and Running Android 2.3 or Below
If you've rooted your phone , you have a few
options. You can install an app like Screenshot UX , which will allow you to take screenshots through a variety of shortcuts, or you could flash a custom ROM that has screenshotting built-in (like CyanogenMod).
How to Take a Screenshot on Android