UserVoice is an online service where customers or your users can give feedback or submit their problems. Allows to create your own forum or a personalized URL that can be used only by your customers.
Users can open tickets and your team can answer, they can use a forum powered by this service for your online business, allows to create a knowledge base where users can find answers.
Is working as a helpdesk system, but also is available to get general feedback from users regarding different aspects that you are interested in (for example, they can propose ideas and other users vote, they can vote on different polls and more).