what the mean exactly about typography ?
Typography is basically the way in which fonts are displayed and arranged online. But really it is an art to design and layout fonts, letters, scale, hierarchy, alignment and much more, based on creating great designs as per the priorities of the page
As web design develops other factors need to be considered like CSS , Cufon and font-face rules.
It is crucial because it influences the users experience and perspective of the page as well as influencing the impact of written content and messaging.
An interesting article here: http://goo.gl/8gJt