Although the requirements vary depending on the state and county, most vehicle registration renewals simply require the owner to pay a registration fee and fill out a form. Some states require proof of insurance to be submitted as well.
Some states also have vehicle inspection requirements. These are sometimes only required during an initial registration, but other states or counties require yearly inspections. If the car's owner has outstanding parking tickets or traffic violations, he may have to provide proof that the tickets have been paid or otherwise resolved to complete the registration renewal process. For normal renewals, many states have online, mail or telephone renewal options that make renewing easy.