Cardinal numbers are primary numbers – that means that they show how many of something there are. These cannot be fractions or decimals, but have to be whole, counting numbers. These include one, two, three, four, five, six, etc.
Ex. Charlie has two (2) cats.

An ordinal number is used to show the order of something, or the place of that thing in a list. These include first (1st), second (2nd), third (3rd), fourth (4th), fifth (5th), sixth (6th), etc.
Ex. This is my second English class.

Hint: a good way to remember the difference is that cardinal is for counting and ordinal is for ordering.