In general, the more information you have about a company the better prepared you'll be to interview and to make an informed decision if you get a job offer.
There are websites where you can read company reviews written by real people. Employees and former employees review the company where they worked, or currently work. Site visitors can read about the company, what it is like to work there, get sample interview questions that have actually been asked by hiring managers, and post their own company reviews.
Do keep in mind that the company reviews on all these sites are posted by individuals, including possibly disgruntled employees. So, use them as a tool to help gather more information about a company, but don't count on them as 100% accurate, because every employee's experience is different.
Company Review Sites
Glassdoor, the company and salary research site, has excellent information for job seekers including company reviews, ratings, salaries, CEO approval rating, competitors, and much more company information. Job seekers can find and anonymously share company reviews, ratings and salary details.
Vault is another source of company reviews. Site visitors can read company reviews, and get the buzz, uppers, and downers, for over 10,000 companies. Company overviews are free, which is all you need most of the time. You'll need to pay a fee if you want more details.
There are also sites which don't contain actual company reviews, but do provide basic company information that's useful for job seekers. Browse the company list on WetFeet.com or search by company name to get an overview and company highlights. For another quick overview, Hoovers.com has a large company directory. Again, you'll need a subscription for detailed information, but the basics are free.
Check With Your Connections
Another way to get inside information is to check who you're connected to at the company on LinkedIn. Ask your connections what they can tell you about the company and what it's like to work for. Searching by company name on Google is another way to find information about companies. Search YouTube as well. You may find company produced videos with information on employment opportunities and company culture.