Hi Hon! 
So many good answers to your question, but here is my two cents  
When i think of someone who is charming i think of a person who is smooth in speaking. confident. Who makes people feel at ease and is pleasant to talk to. While some who flirts is looking for a reaction, be it sexual or not. 
Wow...never thought about it but trying to think about the difference of the two can be kinda hard to explain >.<