An electric field line, a convenient (though imaginary) concept introduced by Michael Faraday is commonly used to represent an electric field. It is to be noted that though the field line is imaginary, the field it represents is real.
An electric field line is an imaginary line or curve drawn through a region of space so that tangent to it at any point is the direction of electric vector at that point.
An electric field line is also the path followed independently by a positive test charge when placed in an electric field. A field line is, in fact, a space curve, that is a curve in three dimensions.
Thought field lines are imaginary, their existence can be shown by sprinkling grass seeds, saw dust or gypsum salt on a glass plate held in the electrostatic field. These particles acquire charges and on gently tapping the glass plate, place themselves along the direction of field lines.