Good for screens, bad for screenprinting!

Corel text and image

The typical computer screen has about 96 pixels or dots per inch (96dpi). At that resolution you would be able to see ragged edges to your text. Operating systems have a little trick to smooth out the edges, they add greyscale to the text. The text appears smoother and easier to read. Here are a couple of comparisons, they've been converted to greyscale, so that they are more relevant to screenprinting, actual screens use colours.

Anti-aliased text

Try squinting at the text, the anti-aliased text looks much smoother.

halftoning in PS, showing postscript banding

This can lead to 'Postscript Banding', If you have an image at 300dpi and you print at 60lpi, you are basically creating a box with dimensions of 5px x 5px. That's 25dots you can add to make your dots. You have 25 (+1 - white) shades of grey. You can see the banding in the image. Anti-aliasing hides the effects until you try to screenprint it!