Offset printing is the age old process of putting ink on paper, where each color ink is is applied to a plate and transferred onto a rubber blanket and then onto the printed surface. Offset printing is very costly and time consuming to set up for and produce the first final print. Once every thing is set up, each copy is relatively inexpensive to print.
Digital printing involves transferring a digital image directly onto the printed surface. Digital printing is more expensive per page but made up for by the lack of costly and time consuming set up.