For each candidate, each voter expresses the utility of that candidates' election to them in the form of a number. In "pure" range voting, each voter may give any candidate any real number, but as the potential for [Strategic voting]? is huge, most systems use some bounds. For example, each voter might give a real number between -1 and 1, or an integer between 1 and 10.
The scores for each candidate are summed, and the candidates with the highest sums are declared the winners.
Tactical voting is pretty much useless, especially in multiple-candidate elections. Any possibility that a favorable result will occur due to inaccurate rankings is overwhelmed by the possibility that a non-favorable result will be more likely to occur.
See also: Approval voting