Summary

Electric field at a point = electric force on a positive test charge divided by charge = \vec E=\frac{\vec F_E}{q} (unit N/C).
Test charge's impact on field is negligible.
Field lines point towards negative and away from positive. Closer lines = stronger field.
Net field from several charges is sum of vectors.
Inside a conductor in electrostatic equilibrium field is zero. Charge sits on surface, surface field is perpendicular to surface.
Inside a charged insulator inside can be nonzero since charge stays instead of moving to surface.