Difference Between Electric Field and Electric Field Strength

To put it simply, the electric field is a region of space in which when a charged object is placed in the field.  It will experience a force acting on it.

However, in Physics, we would prefer a more specific way to define a quantity, and as with gravitational field, we would like to have a definition that allows us to quantify a physics physical quantity.  Hence, we assign an number (and also a direction) to every point in the field.  In this way, we can find out about the force acting on the charge placed at the point in the field if we were to know the charge and the field strength.

The electric field strength is a property of the field and not charge placed there.  Hence, we define the electric field strength at a point in an electric field as the force acting per unit charge on a small positive test charge when placed at that point in the field

Electric field strength at a point in field, E =  F/q

where F is the force acting on the charge q placed at that point in the electric field.

This equation not only defines for us the magnitude of the field strength at a point, but also the direction of the field.  Although the definition specifies a positive test charge, in practical test of the field, we can always use a negative test charge, the q will then be negative.  What it simply means is the field would be in opposite in direction to the force experienced by the negative test charge.

Students also often ask why there is a need to have a small test charge.  This is to ensure the that the test charge itself does not distort the original field significantly.