A flow is a predefined set of campaign destination paths. Flows are unique because they make it possible to have a specific redirect mode for each individual path. Flows often more complex than paths and based on the marketer experience of promoting certain offer.
A path is used to retarget traffic inside one campaign or be a part of the Flow component. The path can use rules or direct URL to guide traffic to a given destination. Usually paths are used to test marketing suggestions to promote a new offer.
A URL destination is used when you want to send your traffic directly to a specific URL without applying any set of rules.