While perhaps a bit "stuffy" to some, the word "logic" is self-explanatory.  For those unfamiliar with its meaning, any good dictionary will define "logic" as something like,

"principles that guide reasoning within a given field or situation".

Logic can be as simple as this:  (a) a coin has two sides, one named "heads", the other named "tails"; (b) you flip the coin and you ask someone, "how did it land?" and the person replies, "it's not 'heads'"; thus (c) it must be tails, as there are only two possibilities, and it is not the first.

Or, logic can be as "everyday" as this:  you ask someone, "is it raining outside?", they reply, "Yes", and you decide to wear a waterproof coat.  You do this because from life's experiences you determine that, "I need to wear a waterproof coat when it's raining because if I get wet then my body's core temperature will lower and the chance of becoming ill will increase".  At some point you analyzed your experiences as to why you got sick and realized it was because you got wet and the water lowered your body's core temperature.  By telling yourself, "if I avoid getting wet when it rains then my chances of getting sick are lower", the reasoning you use is logic.

So, to put it simply, logic is simply reasoning.

Logic that applies to situation "a" may also apply to situation "b"; e.g., you develop a set of reasoning that applies to budgeting your monthly income and you find it also works when you try to give x toys to y children.  So a given "chunk" of logic may apply to more than one situation.  Logicians refer to a multitude of things as a "set", so a given set of logic may apply to a given set of situations, whatever that set may be.

Now let's take it one step further:  what if we were to develop one set of logic that would apply to all given fields or situations?  Sound impossible?  Maybe you think it is impossible.  But if it were possible, then one could say that set of logic was "a set of principles that guide all reasoning".  And applying one or more of the rules of that set of logic could be used to guide one towards a given goal; e.g., "if I want to solve hunger, how do I do it?" If this is indeed possible, that set of logic has as many applications as there are goals.