There are several great applications for drawing on the computer screen, whether using vector or bitmapped graphics and images.
Most images on digital displays are made of a grid of dots of different colors, a technique known as bitmaps or raster graphics. Making such an image have higher quality means using more and smaller dots. Raster graphics can be scaled down with little to no loss of quality, but making a bitmap image bigger makes it look blocky and "pixelated", making the square shape of the pixels especially visible along the edges of shapes and lines. Large image sizes then need large amounts of file space. The larger the image, the more file space the image file will take up. A typical 640 x 480 image requires information to be stored for 307,200 pixels, while an image from a 10 Megapixel digital camera needs ten million pixels (10 megabytes of data) to store the photograph. Digital photographs and video are made up of bitmaps.
In contrast, vector graphics are comprised of collections of paths including points, lines, curves and the angles of geometric shapes such as triangles, squares and more. Because vector-based images are tight mathematical formulas of starting and ending points, they can be scaled to a larger size and not lose any image quality with little to no increase in file size. Increasing the image size of a vector graphic will still have edges that look sharp and smooth. Drawing an image by hand then makes it possible to build up an image of vector based paths that will display and transmit over the web both quickly and elegantly. Explore some of these:
Scalable Vector Graphics
Raster/Bit mapped Graphics
Desktop
Online
Both Raster and Vector Graphics
Professional References
There is always a need to learn a bit more. The Non-Designer's Design Book: Design and Typographic Principles for the Visual Novice by Robin Williams is a recommended resource. It profides some humorous, jargon-free writing that is mixed with design exercises, quizzes, illustrations, and dozens of examples for quick and easy Web work built on basic principles of good design and typography.
Houghton Intro to the Literacy of Image and Audio February 24, 2014 version 1.02