A doodlebat is a picture font. Rather than letters, each character of the font is an image. Doodlebats come with a PDF reference sheet so you can see which image corresponds with each keystroke. Shop for doodlebats.
A doodlebat is installed on your computer the same way you install a font. Please follow the instructions for your operating system and computer to learn how to install a font or go to this tutorial. If you have any questions in how to do this, use a search engine like Google to search "how to install a font".