"An SEC filing is a financial statement or other formal document submitted to the U.S. Securities and Exchange Commission (SEC). Public companies, certain insiders, and broker-dealers are required to make regular SEC filings. Investors and financial professionals rely on these filings for information about companies they are evaluating for investment purposes. Many, but not all SEC filings are available online through the SEC's EDGAR database."  (Source)

10-K is a mandatory annual form filed with the SEC by public companies. It is very long, very detailed, and its format and information requirements are set by the SEC (referring typically to GAAP). (How to Read a 10-K)

An annual report is typically a shorter version of the 10-K that often comes with pictures, nice glossy color pages, a letter from the Chairman/CEO, and an overview of the financials. This report is sent to the company's shareholders each year.  

Annual reports and other company information can usually be found on the company's "investor relations" webpage.

You can also use resources like Hoover's, or the following:

Profiles on public companies in databases usually include a section on financials. Coverage may vary (some include extensive historical data, while others only the most recent data). Annual reports and other SEC filings include financial data as well.