They are parallels.
This also helps answering why two lines of latitude never touch: The lines travel east to west and are parallel to each other.
The longitude of the prime meridian is 0 degrees.
The horizontal line that goes around the "waist" of the Earth is the Equator. The vertical line that goes vertically around the Earth, through the poles, is the Prime Meridian.
The Prime Meridian crosses territory in Antarctica, Europe, and Africa.