When you purchase financial securities, like stocks and bonds, you do this through a stock broker, also called "registered representative", "broker" or "rep". These are employees of a stock exchange member/dealer. Even if you trade on the internet, these "trades" are through a stock exchange member/dealer. Once you decide to trade you need to open a "brokerage account" which allows you to purchase stocks, bonds, mutual funds, and in some cases other types of investment securities using a professional to buy or sell items you designate. Both stockbrokers and financial advisors are licensed by the Financial Industry Regulatory Authority (FINRA), previously called the National Association of Securities Dealers (NASD). http://www.finra.org
There are three major types of brokerage firms: Full Service, Discount, and Online. However the term "online brokers" has blurred, since most full service and discount brokers also provide online services. In order to determine which type of firm is best for you, you need to determine the type of investor you are and what services you need. Whichever you choose, make sure you understand all the costs associated with opening up and maintaining the account. In order to open an account you must be over 18, so if you are under 18 you will have to have permission from an adult to trade on their account.