What is parallel structure in writing?

Parallelism in sentences refers to matching grammatical structures. Elements in a sentence that have the same function or express similar ideas should be grammatically parallel, or grammatically matched. Parallelism is used effectively as a rhetorical device throughout literature and in speeches, advertising copy, and popular songs.
I sighed as a lover, I obeyed as a son. — Edward Gibbon
Reading is to the mind what exercise is to the body. — Joseph Addison
Ask not what your country can do for you; ask what you can do for your country. — John F. Kennedy

Parallelism lends balance and grace to writing. It can make a sentence memorable. But even in prose not destined for greatness, parallelism is important.