Public companies sell stock to the general public. They are required to disclose certain financial information to their shareholders. Therefore, it is easier to locate information on these companies than on privately held companies. Articles that mention annual results, profits, or "stocks" indicate that it is public. If a company is public and Canadian you'll find it in SEDAR. If a company is public and American you'll find it in EDGAR.
Private companies do not sell stock to the public. No disclosure of information is required. Whatever information a private company makes available is completely up to the individual company. It can be challenging to find information about private companies. To quote Forbes: "Private means private."
For clues as to who owns a company and if it is public or private, search:
Some private company information is available at:
Who Owns Whom: North & South America
HG4538.W423 (latest at REF)
Inter-Corporate Ownership (Statistics Canada) 
CD-ROM CA1 BS61 C517 from 1996- (accessible in Documents Data Centre)
Print CA1 BS61 C517 1975-2000 (Documents-Latest in Reference)
Also, search our Business 'Articles' website.
Annual Reports are the primary source of corporate financial information for publicly-held companies. A public company's annual report is available on its website. If the company is private, annual reports will not be available for public distribution.
Also see a company's website: generally 10 years of reports and filings are archived.
Historical annual reports: see the Historical section of this guide.
SEDAR
(System for Electronic Document Analysis and Retrieval)
EDGAR (U.S.) (Electronic Data Gathering, Analysis, and Retrieval system)
International Organization of Securities Commissions, Membership Directory
The North American Industry Classification System (NAICS) is an industrial classification system common to Canada, the United States and Mexico. Gradually, the SIC system will be phased out in favour of the NAICS system.
The Standard Industrial Classification (SIC) is a useful tool for identifying competitors within an industry. SIC codes group companies based on the products and services produced. SIC codes used in directories and databases are generally the U.S. codes. Print versions of the SIC table of codes can be found in the Canadian Key Business Directory or Standard & Poors Directory.
"Beta measures the volatility of a stock relative to the market as a whole, as represented by an index such as Standard & Poor's 500 stock index. A beta that equals 1 means the stock has the same volatility as the market; a beta higher than 1 means the stock is more volatile than the market." (Dictionary of Business Terms by Jack P. Friedman)
Often held after the release of a public company's quarterly earnings, conference calls between top management and investors can be a great source of information. Conference calls are an opportunity for the CEO or CFO to speak informally and candidly about the company's past performance and its future prospects. Informed analysts can ask questions that address their current concerns.
Transcripts of these conference calls are available:
Seeking Alpha Transcript Center