Can someone please brief me on what the "sympathetic" and "parasympathetic" nervous system is?

Parasympathetic and sympathetic nervouse systems are part of the autonomic nervous system. They are the unconscious controllers of internal variables that regulate the flight-or-fight response. The sympathetic system is associated with an increased stimulation and readiness (e.g. increased heart rate, vasodilatation, etc) where the parasympathetic system is associated with the opposite; a relaxed state (e.g. lower heart rate, blood flow directed to accessory organs like the stomach and intestines). They are complementary systems.