Typography is an art form that has been around for hundreds of years. Words and text are all around us every day in almost everything we do. In every piece of type you see, somebody has considered how the letters, sentences and paragraphs will look in order for it to be read by us, or make us feel a certain way when we look at it.
Sometimes it is done well, others not. Often it is us graphic designers who are the ones deciding how it will look, in our brochures, our logos, our websites and so on. The better we are at this, the more effective our designs will be.
Good typography comes from paying attention to tiny details as this can make the difference between graphic design work that is just acceptable or really good. There is more to it than just choosing fonts and making copy look good though - it is also about making things legible and readable (some of most basic functions of good typography) as well as making layouts look good in an aesthetic way.