Sinus tachycardia = heart rate greater than 100 beats per minute (bpm), with each beat's electrical origin in the sinus node.
Most common causes of tachycardia = flight, fright, flight or stress situations (with attendant catecholamine release).
Narrow complex tachycardia = heart rate greater than 100 bpm, with the origin of the electrical stimulus being somewhere (check your cardiac anatomy) above the His bundle (which means, it came from somewhere in the sinus node, the AV node, the atria or anywhere above the His bundle.
An electrical current coming from above the His bundle flows through the normal His-Purkinje system and the QRS will be narrow [less than 120millisecond, or 0.12 sec]).
Narrow complex = supraventricular = above the ventricles.
A re-entrant mechanism is the most common cause of a supraventricular (narrow-complex) tachycardia.
More details may be dug from various cardiac electrophysiology sites and sites such as icufaqs.org.
A simple explanation to your question, in addition to the above comment, is that the term "narrow complex" tachy differentiates it from a "wide complex" such as Vtach. Narrow tachy is different from sinus tachy in that the p waves aren't visible either because the rate is so fast that they are hidden by the t wave or that there isn't a p wave such as afib with rvr.
"Narrow complex" refers to the width of the QRS. One reference uses 120 milliseconds as the "threshold" between narrow and wide complex QRSs. Basically the electrical signal starts "above" the ventricles and as the signal is conducted through the ventricles, it is done so in a normal fashion, so the QRS is narrow. Examples would include sinus tach, a fib, and reentry SVTs (technically, sinus tach and A fib with RVR are SVTs). The treatments are differing for each situation. Here is a (fairly) concise looks at narrow complex rhythms: Narrow Complex Tachycardia
If the arrhythmia originates below the bundle of his (i.e., in the ventricles), the signal starts in the ventricles. Because of the "branches" nature of the conduction system in the ventricles, the QRS becomes wide, hence a "wide complex" tachycardia.
Aberrant conduction throws another variable into the situation, but we can leave that alone for right now.
There are different ways to identify tachycardia. It can be narrow or wide complex;monomorphic or polymorhic, stable or unstable. If it is stable tachycardia it is important to obtain an EKG to determine your plan of action based on the type of tachycardia on the EKG rhythm. Narrow tachycardia has QRS of less than.12 sec such as SVT, at fib. and at flutter. If the tachycardia is stable you start with valsalva maneuver. Nurses are not allowed to do vagal maneuver only the MD in my facility. Then if there is no change in the rhythm you follow this with adenosine 6 mg fast IVP. Sinus tachycardia is usually due to physiologic factor when the stroke volume is compromise that requires an increase in cardiac output such as bleeding or hypovolemia. In this situation, you have to identify the cause and treat the cause. It is a systemic condition and not cardiac in origin and cardioversion is contraindicated. Unstable tachycardia it is straight forward-cardiovert immediately. Please check the ACLS book for updates.