The basic idea behind using a rules engine is to simplify business logic in an application (or in some cases provide cross-application business logic).

The effect here is to remove hierarchical dependencies on logic conditionals and allow each condition (and subsequent action) exist on its own.

A simple example without forward chaining characteristics would be:

standard:

if(A){
  if(B){
    //do something
  }
  elseif (C){
    //do other
  }
}
else {
  //do something else
}

engine:

if(A and B){
  //do something
}
if(A and C){
  //do other
}
if(not A){
  //do something else
}

Yes, this style of logic can already be done in Java or any other language.

The trick comes in the performance of evaluating every possible condition over each iteration and re-evaluating those conditions every time the known data changes.

A useful drools example is forthcoming.