Raster versus vector
Vector image files record images in terms of geometric shapes. These shapes are converted to bitmaps for display on the monitor. Vector images are easy to modify because the components can be moved, resized, rotated, or deleted independently. At this moment Macromedia's Flash is the closest thing to a standard vector format on the Web, and it is not even a real standard.
Standardized Web image formats are at this moment exclusively raster. Every pixel is defined in position and colour and it is difficult to change the content.

Vector image, resolution independent, easy to make changes.	Raster image, fixed resolution.

Resolution
For images to be published on the Web a resolution of 72 pixels per inch is enough, since the resolution of the monitor gives us no need for higher definition. Higher resolutions of the original file will be converted to the 72 pixels needed for display, depending on the fixed size indicated in the HTML page.

Alias and antilias
The pixels in raster images themselves are not curved, they are small squares with sharp edged boundaries. Using polygons to simulate curves in alphanumeric characters or vector graphics makes the edges of objects appear jagged. The strict division between pixels is called alias.

100% size	Linear enlarged, 200%

The technique for smoothing out these jagged edges is called antialiasing, and it usually takes the form of adding pixels of pale colour along the curve.

100% size	Linear enlarged, 200%

This actually makes text seem a little blurred but, strangely enough, more readable.

The "old way" of displaying text on the monitor. No interpolation between the black of the character and the white background, this gives a jagged impression.

Antialiasing interpolates colours where they meet, creating the illusion of smooth non-horizontal or non-vertical boundaries.

When supported on the computersystem (with e.g. the Software "Adobe Type Manager") characters are antialiased. Antialiasing creates unsharpness but better readability.