A typical characteristic of the Alias effect is the stair-like representation of beveled edges within a raster.

While you can scale a vector arbitrarily in any direction, you cannot do so with its representation in a raster. The lower the resolution , the clearer you  will be able to see the single stairs. These stairs occur especially on the edges of image objects. You may also see that smaller objects are only partly represented or are even be completely missing. This particular effect is called the Aliasing effect.

To minimize this effect, Anti-Aliasing methods have been developed. The idea is to reduce the strong contrast between the colors of two neighboring pixels, of which one belongs to the image object and the other does not.

A color tone is set for any pixel to be represented, which is an average of the color tones of its neighboring pixels. By doing so, an optical smoothing of the edges can be achieved.

When exporting a vector graphic into a raster format such as TIFF or PNG, graphics programs, such as IsoDraw, are using an anti-aliasing method. In order to achieve an optimal result, you must choose a sufficient number of colors resp. grayscales for the target format.

Therefore, it is recommended that you choose a size and resolution during the raster export that which will be needed for the subsequent application, (for example, use on a web site).  

Furthermore, you should keep in mind that when enlarging raster graphics on which an anti-aliasing method has been applied, the result will become diffused.