SpellGuard is a program that interactively checks your spelling i.e. as you type. After a mistake is found, if requested, it will attempt to give you matches from the dictionary of over 56,000 words.
SpellGuard is quite simple, it starts checking your spelling as soon as you start it up. When you make an error in spelling, or you spell a word that is not in the dictionary, the SpellGuard window will float to the top of the screen, but it will not interfere will your typing. At the next keystroke, SpellGuard will float back out of sight (these settings can be changed).