2 Answers

WHAT IS A PROOF OF SERVICE?
The Proof of Service is a document that lets the court know that a party was served with court documents. It is filled out by the server after the documents have been served. The proof of service is then filed with the court in most situations. Many forms have their own Proof of Service form. If you are serving a form that has its own Proof of Service, you will need to use that Proof of Service form. Common documents with a specific Proof of Service forms include:

Summons for most types of cases
Civil Harassment, Domestic Violence, and Workplace Violence Restraining Orders
Subpoenas
Many of the Probate, Family, Juvenile, and Small Claims forms that require personal service also have their own Proof of Service forms.

The definition is here

http://www.lectlaw.com/def2/p210.htm