Many Windows applications come with a large amount of clip-art which is copyright free and may be used in documents, charts, presentations, etc. Bitmaps store data about every single pixel in the image. The larger the image, and the higher the colour resolution, the bigger the resulting file size is. However, this format is not very efficient and results in large file sizes. JPG images can have several different levels of compression, but note that the higher the compresssion used the poorer the image will look.
Icons are similar but normally much smaller physically in size.
They can be used for application icons displayed on the desktop or used for toolbars within applications. Meta-files store graphical information in a totally different way to bitmaps. Instead of storing each and every pixel which can take up a lot of space, vector graphics only store mathematical formulae which describe the picture. Take a very simplistic house for example, you could draw the main bit using a rectangle and then add a triangle on the top. A meta-file would only store where these two shapes are on the image, their dimensions and colour.