What is the difference between a Wren and a Sparrow?

Wrens have a long, slender, slightly curved beak, small oval body and stiff tail.

The House sparrow has a short, thick bill for eating seeds, and short legs.

http://www.allaboutbirds.org/page.aspx?pid=1054

Wrens and sparrows come from two completely separate families. Wrens (New World, Troglodytidae) primarily feed on insects. https://en.wikipedia.org/wiki/Wren They are a pleasure to have around, in my opinion.

True sparrows (Old World, Passeridae) are seed eaters. https://en.wikipedia.org/wiki/Sparrow American sparrows are from a different family, Emberizidae. Sparrows will readily attend birdfeeders.

House sparrows are Passeridae. Their diet is also primarily grains and seeds. Unfortunately, they were introduced to the U.S. from Europe in the nineteenth century and are considered an agricultural pest. They've thrived in many parts of the world due to their strong adaptability. In some areas it's legal to destroy them.