The line in the center of a soccer field is the center line.
half way line
The goal lines are the shorter lines on the field and the goals are on them.
The touch lines are the longer lines on the perimeter of the soccer field. Specifically, the ones without the goals on them.
Whenever the ball exits the field across a touch line.
When it completely exits the field across a goal line or touch line.
The sidelines of a soccer field (the longer edges that do not have goals centered on them) are called "touch lines". A ball that has wholly traveled over the whole of the line is said to be "out of touch".