What is the difference between EFAS and FSS?
EFAS - Inflight Weather Advisory Services (aka Flight Watch) is a special frequency in the United States on 122.0.  It uses remote communication outlets (RCO) and is typically collocated with navigational equipment.  

When you talk with a FSS weather briefer on EFAS you are talking with a more experience weather briefer who can provide a greater degree of accuracy with changing weather conditions.  When I took a tour or the Prescott FSS station after Lockheed Martin took over, the tour guide state the EFAS station is manned by a Level 2 weather briefer who is more knowledgeable than a Level 1 briefer who you might talk too over a regular FSS frequency.

FSS personnel can talk to pilots over RCO and the frequency is typically found along the top of the VOR dialog box on sectional charts as well as 122.2 and 122.4.  These briefers are available to open and close flight plans, accept PIREPs and provide basic weather services.

The picture below shows that you can talk with McAlester FSS on 122.1 and receive their response over the VOR 115.2.