A hermeneutic is an assertion (including presuppositions and deductions), or system of assertions, both substantive and methodological, used to interpret and understand a document.

This is a mouth full.  But it can be best illustrated by comparing and contrasting a hermeneutic to a theology.

Many propositions or systems can be both a theology and a hermeneutic.  The distinction is primarily that a theology is an assertion of truth deemed important enough that, even when not used to intrepret Scripture, the proposition(s) may impact faith and practice.  Consider the following example:

The Trinity is both a theology, and a hermeneutic.  As a theology, we worship a Triune God.  Even when not reading a passage of Scripture, this doctrine rests at the core of one's view of God.

Against this, there is also a trinitarian heremeneutic.  That is, in the account of Jesus baptism, we see all three members of the Trinity mentioed.  The Trinity therefore forms a useful doctrine for intrpreting this passage.  A more vivid illustration is the intrepretation of our Lord's death.  If we adopt a "modalistic" hermeneutic (one person, one God), Jesus was offering His sacrifice to Himself.  If we adopt a Trinitarian hermeneutic, Jesus was offering Himself to the Father.

Other "substantive" propositions or systems that can be both a doctrine and a hermeneutic are Calvinism and dispensationalism.  For example, a Calvinst may hold the doctrine of "irresistabel grace" as foundational to his or her concept of God.  In this sense, it is a theology.  However, when this  proposition is used to interpret a passage of Scripture, it is a hermeneutic.  Consider the following example:

Ephesians 2:8-9 teaches:

"For by grace are you saved through faith, and that not of yourselves, it is the gift of God, not of works, that any man should boast."  Imposing a Calvinistic hermeneutic on this verse would be to interpret it as teaching:  "You are saved by faith.  But you cannot believe in and of yourself.  God must provide you 'sovereign' grace to empower you to believe."

The point here is not to visit the debate(s) surrounding Calvinism, but simply to illustrate how a theology can operate as a "hermeneutic" in the intrepation of a passage.

It is instructive to note that one may hold a theology, but not regard it as a useful hermeneutic for interpreting a passage.  For example, a Calvinist may observe that by the rules of Greek grammar, the "that" which is not of ourselves is, in this passage, not 'faith,' but 'salvation.'   This does not mean that the Calvinist is denying the theological propositions of Calvinism.  This is only to say that, as a hermeneutic, the doctrines of Calvinism are not relevant to the interpretation of this particular verse.