On Windows, you can take a screenshot of the currently active window by pressing Alt-PrntScrn (aka Alt-PrintScreen, hold down the Alt key, press PrntScrn, release both) and the screenshot is put in the Clipboard, open up any image program (I like IrfanView best, get it on http://download.cnet.com) or even Word or WordPad, press Ctrl-V to paste. To get the entire desktop (everything showing on screen) press PrntScrn instead of Alt-PrntScrn.
To capture menus, including right-click menus, it is usually easier to grab the entire screen (press PrtnScrn) and then crop the captured image.
On OS/X there are several keystrokes, to get different portions of the screen. The keystrokes put the screenshot into a PNG file on your desktop (PDF in Mac OS 10.3 and earlier). If you want it put in your clipboard instead, add holding down the Ctrl key also, e.g. Command-Control-Shift-3. (Note: http://www.osxkeyboardshortcuts.com/keyboard-symbols.html has a chart of keyboard symbols and the easy-to-type names used in documentation.)
Entire Desktop: Command-Shift-3
A portion of the Desktop: Command-Shift-4, position the mouse at one corner of what you want to capture, click-drag to the opposite corner, when you release the mouse button the area will be captured.
A specific application window: Command-Shift-4, space, move the mouse pointer over the application window you want captured, click the mouse.
To include the mouse pointer or a menu in your screen shot: use the Grab utility by selecting Dock > Utilities > Grab. Set the Grab Preferences to use the cursor icon you wish to have in your screenshot.
iPad, iPhone or iPod Touch
Press and hold the menu button (the main button below the screen), press the power/lock button (the switch on the top edge of the device). The screen will flash white and make a camera shutter noise, to indicate the shot has been taken. Release both buttons (holding too long will give the power-down confirmation). The screenshot is saved in the default Photos app in the “Saved Photos” or (depending on your device) “Camera Roll” album. Email the photo by clicking the Menu icon (rectangle with arrow coming out of it, bottom left corner of screen). Or, click the Menu button from an index page to select multiple pictures to email.
Different devices use different buttons, most use one of these button combinations: Power + Home, or Power + Volume Down. Mine requires holding down the Power + Volume Down for several seconds until the screenshot appears half-size on the screen.
Open the Gallery application, the file is in the Screenshots album. In Android 4.2 and later, your screenshot will appear in your notication bar, to open it in Gallery or to Share it.
There are apps for screenshots, make sure an app specifically supports your device. Generally they give you more control over your screen shot than the built-in screenshot function.
You can connect your Android device to your computer, take the screenshot with your computer, see http://www.wikihow.com/Take-a-Screenshot-of-an-Android-Device-%28from-a-Computer%29