What are Heavy Metals?
Heavy metals are constituents of the earth's crust, naturally-inherent since the time of earth's formation. Heavy metals can also be discharged by industry, in addition to their natural sources.
As it relates to the safety of the food we eat, the questions surrounding heavy metals is less about whether or not they are present. Instead, the questions become:
1. How do heavy metals become present? More specifically, what percent of heavy metals are sourced from industry versus nature?
2. In what amount are they present? This translates to:
a. What are the established tolerance limits (safety thresholds)? and
b. How does the amount present compare to that limit (how real is the threat) in the food we eat?
The technical definition for heavy metals is "metals with a density greater than a certain value, usually 5 or 6 grams per cubic centimeter."
Heavy metals (in relation to food safety), refers to those metals of high density, regardless of their source, as being hazardous. Not all hazardous items are metals, and so the term "hazardous elements" is used where both metals and nonmetals are described. Further, not all hazardous elements are likely to be found in food products. 
To better explain, let's take a look at the periodic table of elements (see Figures 1-4):
Figure 1. Periodic Table of Elements, Categories: The shaded portions represent three categories of elements: metals (light orange), metalloids (light green), and nonmetals (light blue). 
Figure 2. Periodic Table of Elements, Density: The portions outlined b