3D printing is an umbrella term for various additive manufacturing technologies. The underlying principle for all these technologies is the same, create objects by depositing material layer by layer but the way the principle is used to create objects varies across these various technologies.  

(a) Fused Deposition Modeling (FDM)

FDM, or Fused deposition modeling was invented in the late 80’s. Using this technology, drops of molten thermoplastic materials are joined together to form a shape. As the material hardens, 3D object is formed. Please check the video below to see how FDM technology works

(b) Stereolithography (SLA)

This method was invented in 1986 and was considerd the first 3D printing technology to be invented. Printers using stereolithography concentrate the beam of UV rays on the surface of the object to be replicated. The object is filled with resin (photopolymer). When light hits the resin, you get a high resolution 3D model of the object you have used. Below video illustrates SLA technology in detail 

(c) Selective Laser Sintering (SLS)

(d) Selective Laser Melting (SLM)

(e) Electronic Beam Melting (EBM)

(f) Laminated Object Machining (LOM)

(g) Polyjet Printing (PJP)

(h) Multi Jet Fusion (MJF)