Joshua Porter writes about the use of microcopy to create confidence and alleviate concerns in customers about the actions they take on your site.

Ironically, the smallest bits of copy, microcopy, can have the biggest impact. ... Microcopy is small yet powerful copy. It’s fast, light, and deadly. It’s a short sentence, a phrase, a few words. A single word. It’s the small copy that has the biggest impact. Don’t judge it on its size…judge it on its effectiveness.

Porter provides a few detailed examples showing how microcopy helped users entering ecommerce funnels, in one case reducing errors by qualifying an address form, and in another case adding information to assure customers that they don't need a PayPal account to purchase with PayPal. He provides a few more before/after bits of copy for typical registration and ecommerce scenarios as well.