Aliasing is what happens when a computer graphic -- which is built in discrete units, called pixels -- doesn't have high enough resolution to reproduce the intended image. It has nothing to do with a "continuous edge," but how fine details are mapped onto pixels on the screen, making edges either look smoothe or jagged.
Anti-aliasing is the product of a number of techniques (almost always algorithmic, i.e. not done by hand on a pixel-by-pixel basis -- at least in modern graphics programs) that minimise jaggedness/distortion found in graphics files. A couple good examples can be found
here (ignore the technical mumbo jumbo if you'd like, just look at the pics in "Examples") and
here.
Often the easiest way to achieve an anti-aliased design is to simply start much larger than the finished design. Shrinking the image, when done correctly,
should minimise any aliasing present. That requires a program like The Gimp, Photoshop, or Paint Shop Pro and the settings required vary between programs. Be sure that when you resize the program is "resampling" the image -- methods such as "bicubic" or "smart size" work best. If you let us know what program you're using, someone might be able to get you headed in the right direction.
Another option is to use vector graphic tools to do your drawing instead of your computer mouse or graphics tablet. Vector tools usually come built-in with anti-aliasing options. There is also a great Web application called Vector Magic, linked in the
2D Tutorials & Resources sticky. When vector tools are combined with the "work large, finish small" principle, you're pretty much guaranteed a good and anti-aliased final product.
Hope this helps clarify what FLS means when they say your design is aliased, and what you need to do to fix it.
