I assume you mean extracting an object from a flat image? There are several ways to do so, but the easiest is to use the Pen Tool.
Draw around the object. Get as much detail in as possible, then make the Vector line a Selection, and Cmnd+J to make the selection a new layer.
Alternativly, duplicate the layer and put a layer mask onto the new one. Hide the lower image so you can see how your editing is going. Mask out the unwanted areas and keep the object you want visible. For fine details (such as hair or grass), use a smaller brush, and switch between black and white to get as much of the detail in as possible.
I'd suggest using a combination of both. Set up the mask, then get the main countours and shape of the object with the pen tool, bulk around fine details, then zoom in close and brush them in/out.
