What is the difference between an aldose and a ketose?

An aldose contains an aldehyde group; a ketose contains a ketone group,
The ending -ose tells you that both aldoses and ketoses are carbohydrates.
Carbohydrates are really just polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses).
So, for a carbohydrate to be an aldose it needs an aldehyde group and, to be a ketose, it needs a ketone group.
In the structures below, the aldehyde and ketone carbonyl groups are marked in pink.
Examples of aldoses are glucose
and ribose.
Examples of ketoses are fructose
and ribulose

(from 2012books.lardbucket.org)