You can edit pixels over an entire canvas or select one of the selection tools to constrain your editing to a particular area of an image:

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The Marquee tool selects a rectangular area of pixels in an image.

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The Oval Marquee tool selects an elliptical area of pixels in an image.

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The Lasso tool selects a freeform area of pixels in an image.

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The Polygon Lasso tool selects a straight-edged freeform area of pixels in an image.

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The Magic Wand tool selects an area of similarly colored pixels in an image.

The pixel selection tools draw selection marquees that define the area of selected pixels. After you draw the selection marquee, you can manipulate it by moving it, adding to it, or basing another selection on it. You can edit the pixels inside the selection, apply filters to the pixels, or erase pixels without affecting the pixels beyond the selection. You can also create a floating selection of pixels that you can edit, move, cut, or copy.