Computer monitors display images by lighting up tiny dots spanning the screen's entire surface. Unless you're using a magnifying glass and can actually see the individual dots, these dots come together to form images, text, or whatever else is painted on your screen. This is very similar to impressionistic painting with many painted dots making up an image. The image gets clearer as the distance between you and the painting is increased, but becomes a series of unrelated dots as you get closer and closer. The big difference is that all the dots on the computer monitor are exactly the same size and much much smaller than you see on an impressionist's work. Also, these dots are not round as you might think - they're square. The proper term for each tiny square, when speaking in relation to monitors, is called a pixel. Since the pixels are square, when you draw curves and diagonal lines, you get what is called the staircase effect like the image below on the left.

As you can see, curves and diagonals tend to look bad or pixelated, while vertical and horizontal lines look fine. However, there are a lot of tricks that you can use to make the illusion of a smooth curve or diagonal. One of those tricks is called antialiasing, which is shown in the image above on the right. It gives your eyes the illusion that a curve is smooth by using different colors and shades of pixels, although the pixels are still square.

Fortunately, most image editing applications such as Adobe Photoshop give you the ability to draw either aliased or anti-aliased lines and text, and also give you the ability to scale images proportionally while anti-aliasing the entire image in the process.