Eraser Tool is the basic tool for turning drawn pixels back into transparency. It works for both tiny one-pixel cleanup and larger erasing with a broader brush.
Icon

This is the eraser icon. It sits near the bucket tool.
Effect
It makes pixels on the active layer transparent. By changing brush size and brush shape, you can clean unwanted edges or carve the shape down more precisely.
Shortcut
The default shortcut is Shift + E.
Related Shortcuts
Ctrl/Cmd + Mouse Wheel: change brush size- Hold
Space: temporarily pan the canvas Alt+ click: temporarily switch to the eyedropper
How to Call It from Other Tools
You can switch to it with Shift + E. Also, if you are already using the Pen Tool, holding E temporarily turns the pen into an eraser, which makes it easy to bounce back and forth between drawing and deleting.
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