Credit unions are financial institutions, like banks. They offer many of the same products and services -- savings accounts, checking accounts, ATM/debit cards, credit cards, auto loans, mortgages, home equity loans, etc. Both have deposit insurance so your money will be safe.
The main difference from your viewpoint as a consumer is that you can become a customer of any bank, while credit unions can only serve people who fit in their "Field of Membership." Some credit unions are sponsored by a company and only the employees can join. There are numerous credit unions that have "community charters" which allow them to serve people who live within a defined geographic region (i.e. city, county, zip codes, etc.).
Credit unions are financial cooperatives which means people deposit money in savings accounts and loan it to each other. It's kinda like Baily Building & Loan from the movie It's a Wonderful Life. There are no outside investors, nor are there stockholders. Check out wikipedia for the history of credit unions and financial cooperatives.
Hope this helps. Good luck!