Pleadings, motions, discovery and other case documents are required to be "served" on parties in a lawsuit. This simply means that everybody gets a copy of everything. There are rules that govern how these things are "served" or, in plain English, are sent. The rules differ by state and court system. Here are links to the California rules:
Electronic service (Cal. Rules of Court, Rule 2.260)
Direct filing of document in court (Cal. Rule of Court, Rule 2.304)
Requirement for signatures on documents (Cal Rule of Court, Rule 2.305)
Service by fax (Cal Rule of Court, Rule 2.306)
"How to Serve a Business or Public Entity (Small Claims)" - by the California Judicial Council