First of all it is important that we define the context. If a company says "We are planning to integrate Artificial Intelligence software in this business process", What they often mean is "we want to automate this process using a software robot". If this is the context, RPA and AI can mean the same thing.
In all other situations:
RPA solutions automate processes using "software robots". These robots can be very simple, e.g. a click-robot that copy pastes information from an excel sheet into a database GUI. The more advanced RPA solutions use machine learning to observe and learn how a human performs a task and then automating it. Artificial Intelligence describes a field of study that aims to automate human tasks such as decision making. To get to your question: the difference is that RPA can use artificially intelligent algorithms to perform complex tasks that need some degree of "reasoning" or "cognition". So there is an overlap between RPA and AI. Not all RPA solutions are intelligent, but the ones that are, use Algorithms that are used in AI as well.