The verbs bargain and haggle are similar. To bargain is to negotiate price. As an example, if you want to buy a car, you say, "What is the best price?" You discuss the price with the other person.
To haggle is to bargain with more effort. The verb haggle also has a negative connotation in Western culture, sometimes. It defines when someone is bargaining to the point of annoying the other person. In Western culture, it is acceptable to bargain on some items. However, it is not appropriate to bargain on every item. Haggling is the act of bargaining when it is not considered necessary or appropriate.