How are heredity and evolution interlinked?

Heredity is the passing of traits from the parent organisms to offsprings. The traits thus acquired are known as inherited traits. These are different from the acquired traits, which are a result of the particular environment in which the organism exists. For example, the color of eyes is an inherited trait, whereas the language spoken by a person is an acquired trait (and can be different from his/her parents). 
Evolution is the change in these inheritable traits of species over successive generations. The traits that allow organisms to better survive and reproduce, if passed from generation to generation, will enable the particular species to survive over generations. Changes in these particular inherited traits over generations, allowing better survival and reproduction chances, will result in the evolution of species. 
In other words, evolution is linked to the heredity through inheritable traits. 
Hope this helps.