What is the difference between the Ford model t and Ford model a?

They were totally different vehicles. The Model A had twice the power, used convention drive controls as you see on today's vehicles and was nothing like the Model T. You can find out more by clicking the link.

The model A was the first car Henry Ford put into production. After that he used the next letter of the alphabet for each model he created. Model B, C, D, ect. The model T was the car he was able to mass produce and sell for price most could afford. His son Edsel Ford designed the restyled 1928 Ford. Since automakers were not using name for there product yet Edsel Ford decided to start with the first letter of the alphabet again. In 1928 Ford reintroduced the Model A again. As a result of his decision Ford produced 2 Model A's. The first in 1903 to 1904 and the second from 1928 to 1930.