The eraser tool has been modified a bit in Photoshop 5.5. The standard eraser 
allows you to adjust the brush size and opacity. The Magic Eraser 
works well on flat images as it allows you to erase any area of the same or similar color. You can set the tolerance to different levels and experiment to erase a section of an image by just clicking in that area. For the Magic Eraser, check the Contiguous box to erase only the similar pixels that are touching each other. Uncheck Contiguous to erase all pixels in the layer that are that similar color.
The other eraser choice is called the Background Eraser 
. It will not just magically erase the background of a photo(too bad), but there are some settings that allow you to erase only certain portions of an image. You will get an opportunity to play around with erasers during Exercise PS.
Despite the additions of new eraser tools and a whole new feature called Extraction(Image...Extract or Alt.+Ctrl.+X), I have found the best way for me when I want to remove a section of an image is to magnify it, use the Lasso tool 
and either copy and paste it into a new tranparent image or Select...Inverse and delete everything else. Remember that only GIF format supports Transparency so if you want to make a JPG image appear transparent, you have to make the background of the image the same color as the background of your webpage.
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