Thank you for the opportunity to help you with your question!
Political parties put forth their efforts to win in elections against their opponents. Their efforts or position within society may constitute wide array of opinions on different issues while interest groups tries to focus on specific issue so that they can effectively influence policy makers or management. Political parties are intended to make and operate government while interest groups share common agenda or goals without having intention to make government or win against any kind of opponents. So, one can say there are differences in internal politics, purpose and method of accomplishing goals.
Please let me know if you need any clarification. I'm always happy to answer your questions.