If you arrive at Narita airport, primary gateway to Japan, and going to take train, you will use either Narita airport terminal 1 station or Narita airport terminal 2・3 station depending on which terminal you are going to arrive at. Both stations are served by two railway operators, JR East and Keisei. If you are a Japan Rail Pass holder, take JR train but if not, Keisei is cheaper.
Then, Keisei operates two lines, Keisei main line and Narita Sky Access line. Limited express train with extra supplement, Skyliner and Access express bound for Haneda airport run via Narita Sky Access line. However, if you want to go to downtown Narita, take Keisei main line train.
Especially if you are arriving at terminal 2 or 3, you need an extra care not only which line to take but which platform to wait. When taking Narita Sky Access line, you have to pass the ticket gate once and take a train from track number 1. On the other hand, if you want to take Keisei main line, you have to pass another ticket gate later and take a train from track number 3. However, track number 1 and 3 are actually on the same platform. Track number 1 is located at the rear side of the platform and track number 3 at the front part, just small gate dividing it.