What is the difference between activity and reactivity when discussing elements? I screwed up on a true/false question that stated that lithium is the most active alkali metal. I answered false, because I remembered that the lower you go in this group, the more explosive the element is. 

It seems, however, I was thinking of reactivity and not activity. I know about the activity series, and it's something I wouldn't like to memorize, but I would a deeper explanation on the difference between activity and reactivity. Why do we have these two terms?
Activity is an ordered series, showing which element will replace another element. For example, in a solution of copper sulfate in water, a piece of iron quickly becomes coated with copper as the iron displaces copper from the solution because iron is more active. No explosion took place.

$\ce{Fe(solid iron) + CuSO4 (dissolved in water) -&gt; Cu(solid copper) + FeSO4 (dissolved)}$

Reactivity is less clearly defined. In general, it means how easily some chemical change takes place. For example, chlorine, oxygen and sodium are all very reactive, easily attaching to other elements or replacing elements in compounds. Argon and helium are not reactive, and rarely, if ever, combine with other substances. Nitrogen triiodide, $\ce{NI3}$ and acetylene, $\ce{C2H2}$, are very reactive, but in a way opposite that of sodium or oxygen: rather than attaching to other substances, forming molecules, these compounds want to break up their molecules. In fact, even touching $\ce{NI3}$ will make it decompose violently... Now there's your explosion.