Well, really there is no difference as far as licensing goes. Licensed realtors may be (and most are) listing agents, buyer's agents, etc. Some states have a different or additional license for commercial realty.
All realtors also take continuing education throughout their career. Some classes are mandatory for us such as law reviews, ethics training, forms review but there are many designations that we may obtain and that's where you usually see where that particular agent might focus their time. Some National Assoc. of Realtors designations are Accredited Buyers Agent (ABR), Short sale & foreclosure (SFR), Graduate, Realtor Institute (GRI)
REALTOR® Professional Designations & Certifications
You might begin your search for an agent by talking to friends who have sold recently, drive around your area and see whose signs are up on for sale properties, then interview several agents to find a good fit for your situation. Some questions you want to ask are, what are your marketing strategies, how/where do you advertise, are you a member of the local MLS, and remember sometimes the best agents are from small companies where that agent and their broker and/or an assistant personally handle all levels of your transaction.
Who was your agent when you bought the home? Many people go back to previous agents because they did a good job the first time and trust has been built.