Described qualitatively the path of the charged particle moving in uniform electrostatic

(i) parallel to the field

( ii ) Perpendicular to the field

(iii) at any arbitrary angle with field direction.

(i) The charged particle moving parallel to electrostatic field experiences a force in the direction of field. Therefore its path is a straight line.
(ii)
If charge particle enters perpendicular to field,it experiences electrostatic force in a direction perpendicular to initial velocity.
Therefore its path will be superposition of two straight line paths:
one along the direction of initial velocity and other perpendicular to initial velocity. Therefore the resultant path will be parabola.
(iii) 
When charged particle enters the electrostatic field making an angle with it,then the velocity of charged particle may be resolved into two components:
One along the field and other perpendicular to the field.
The charged particle will experience electrostatic force along the direction of field and will continue to move perpendicular to the field.So the path is superposition of two straight line paths,which is parabola.