The main difference is that tourists usually stay in a place for more than 24 hours, go back to their home country once they're done and their purposes are purely recreative: They travel for pleasure, family, fun, although sometimes even for business. They all end up staying overnight and usually go to follow a long itinerary or to see monuments or do some longer sightseeing.
On the other hand excursionists are considered as one-day visitors because generally their trips do not include an overnight stay and they also have a specific purpose (e.g., they have a specific itinerary, or place to see, they are hiking or cycling somewhere or seeing a place for a limited amount of time, like 4 hours.)
This is a small chart that might make things clearer:
Hope this helps!