You can erase from any area of your photo using the Erase Brush Tool. Erased pixels become transparent.
When using the Paint Brush Tool on a RAW or image layer, a new mask layer is created with the strokes applied onto it. This aids non-destructive, layer-based workflows whereby the newly created layer can be switched off from view to investigate the effects of erasing quickly.
The following settings can be adjusted from the context toolbar: