"Prior art" refers to any previous mention of the technology or device in the public domain.
An inventor who believes their invention is new and wants to apply for a patent must first conduct prior art searching, before applying for the patent.
In other words, the inventor must search the internet, research databases, and other sources beyond granted patents.
It is not an exact science, so it is difficult to guarantee that one has successfully searched all relevant prior art and citation.
It is important to understand the concept of what the invention aims to do, what is novel about it, and what the key features are.
Types of searches to do:
Keyword search
Product Search: Good idea to find out what is already out there.
What to search:
old technologies
products not already on the market
trade/industry resources: magazines, catalogs, websites, etc.
Shops, books, periodicals, etc.
Also a great idea to talk with others with long experience in the field
Patent Searching: this can last from minutes to several hours
When searching patent databases, don't just do keyword searching. Also do: Classification search; Assignee-based search; Inventor-based search
Remember:
Search different patent databases and non-patent databases
Keep a record of everywhere and everything you search
Source: European Patent Office. Inventors' Handbook. Novelty and Prior Art. 2008