This article explains what the "Grab" item in the Apple menu is and how to use it.

Selection Modes

Grab provides four separate ways to grab images from your screen. Whenever you choose one of them, Grab will display a panel on the screen to allow you to proceed or cancel. This window is always removed before you take your snapshot.

Selection

This option is used to grab arbitrary areas of the screen. When this option is selected, your mouse cursor will change to a cross-hair type shape. Move the mouse cursor to the area on the screen where you want to capture an image. As you drag the mouse, numbers will appear to the left of the cursor, indicating the current screen coordinates of the mouse. Once you are positioned properly, drag the mouse forming the rectangular area you wish to grab. Once in this mode, the numbers to the left of the mouse cursor indicate the size, in pixels, of the area you are about to grab. When the rectangular area is the correct size, release the mouse button, and a window will appear, showing you the image you just "grabbed". At this point you may save the image, or copy it to the clipboard.

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This option is used to grab a particular window from your screen. Once Grab brings up its panel, you should position your windows any way you like. (In the case of windows like torn off menus, which are only visible when their application is active, you might need to switch applications here). When you are satisfied, click your mouse on the "Choose Window" button. The camera icon in Grab's panel will show that it is ready to take a "picture", and your mouse cursor will change to a small camera, or one of the cursors you have chosen from the Preferences panel. Move the mouse cursor to the window you want to grab, and click on the window to "grab" it. This mode will correctly capture a window even if it is partially obscured, or partially off the screen. If you click on the screen background and not on any window when in this mode, the whole screen will be "grabbed."

Screen
This option is used to grab your whole computer screen. Once Grab's panel comes up, click anywhere outside the panel to take the screen shot. The Grab application will always hide itself before "snapping" the picture, so you do not need to move Grab windows off the screen.

Timed Screen

This option is used to grab your whole computer screen after a 10 second time interval. When you are ready to start the timer, click on the "Start Timer" button in Grab's panel, and the camera icon will start counting the ten seconds. During the last second, or so, Grab will sound an alarm indicating that the actual grab is imminent, and then the full screen will automatically be "grabbed" (as in the "Screen" option). This option is useful in order to capture popup menus, button states, etc.
Note : Once you have grabbed an image you can always re-grab smaller pieces of it right out of the grab document window, using the Selection method.

Grab File Format

The Grab application saves files in the .tiff format. Files saved in this format are typically larger than those saved in .gif or .jpeg formats. Images saved in the .tiff format can be converted to other formats which may be used on the web, in a word processor and so forth.