Anti-aliasing produces smooth-edged type by partially filling the edge pixels. As a result, the edges of the type blend into the background.

Anti-aliasing set to None (left), and Strong (right)

When creating type for display on the web, consider that anti-aliasing greatly increases the number of colors in the original image. This limits your ability to reduce the number of colors in the image and thus to reduce the size of the image file. Anti-aliasing may also cause stray colors to appear along the edges of the type. When reducing file size and limiting the number of colors are most important, it may be preferable to avoid anti-aliasing, despite the jagged edges. Also, consider using larger type than you would use for print. Larger type is easier to view on the web and gives you more freedom in deciding whether to apply anti-aliasing.