Yeast is a single-celled organism called Saccharomyces cerevisiae. To grow and live, yeast needs food, warmth, and water. It ferments its food, sugar and starch, to make carbon dioxide and alcohol. What makes baked goods rise is carbon dioxide.
If you want to use a starter instead of commercial yeast in a recipe that calls for yeast, you can use 1 envelope of active dry yeast instead.