Day
A day is the time it takes for one complete rotation of the Earth on its axis relative to the Sun. It is commonly divided into 24 hours, each containing 60 minutes, with each minute made up of 60 seconds.
Due to irregularities in Earth's rotation the length of a true solar day varies slightly so a mean solar day is used for consistency.
Hour
An hour is made up of 60 minutes which in turn is made up of 60 seconds. There are 24 hours in a day.