The SMAW referred to above is also called MMA (Manual Metallic Arc). MMA is an older designation, as is MIG (now GMAW), TIG (now GTAW), and I'm sure the oxy-fuel processes and others have changed some over time as well. Big pipe (Think Alyeska pipeline) can have long straight sections welded using SAW (Submerged Arc), but that is a pretty heavy-duty high-deposition industrial process. Only good directly (or very near directly) downhand.
For learning and understanding, I think GTAW (TIG) is great. I don't know how to use this process personnally, but all the best welders I've known over a 40 year career in manufacturing have been good TIG welders. MIG is easiest. MIG and FCAW (Flux Core Arc Welding) use very similar equipment and are forgiving processes when starting out.
Best of luck to you.