These two devices are similar in many ways. I would say trim routers are a narrower variety of regular ones. They are used for edging and chamfering products. It is possible to process not only straight but also curved surfaces equipping a trim router with attachments of various types. In particular, using such a device and corresponding bits, you can make skirting boards and process curly coffers. You can also do similar work with ordinary routers, but they have more functionality and, therefore, are universal.