What is the difference between reductive and non-reductive disaccharides?

Reductive disaccharides have a free aldehyde or ketone group, which gives them reducing power.
Non-reductive disaccharides do not have a free aldehyde or ketone group, and therefore do not have reducing power.
Examples of reductive disaccharides are maltose and lactose, while an example of a non-reductive disaccharide is sucrose.