Hi Shelly, It is important to know, especially if you are growing tomatoes in a container, or plan on preserving the harvest (canning tomatoes.)
Determinate tomatoes grow to a certain height and no taller. They usually produce their fruit all at once or in a couple of weeks. Thats good, especially for commercial producers, who want the harvest all at one time for canning.
Indeterminate tomatoes, like most heirlooms, just keep growing and producing all season and until the first frost. These plants get big and heavy with tomatoes, so early staking and providing support is important.
I wrote about indetermanant and determinate tomatoes in a hub what Is An Heirloom. http://hubpages.com/living/What-is-an-heirloom-tom...