FFF stands for "Fused Filament Fabrication." This type of printing is also referred to as FDM, or "Fused Deposition Modeling." This means that the printer melts a filament to deposit onto the print bed, forming the object to be printed.
Most inexpensive home 3D printers are FFF-style, but there are many methods of 3D printing, including SLA (stereolithography) and SLS ("Selective Laser Sintering"). In general, unless you want to spend big bucks, you want FFF.
SLA has advantages, but requires special photosensitive resin in place of a filament. These resins are expensive, and limit your choice of materials.
SLS printers are cool -- after all, they use lasers and they can print in metal! But, an entry-level machine is about $10,000 USD (e.g., the Sinterit Lisa), and higher-end machines start at more like $100,000. Compared to $300 - $500 for an entry-level FFF printer, you can see why most home users are choosing FFF.