There was a thread on here that tried to explain this but for some reason I can't seem to find it.
The gist of it was as follows:
On the cultural victory screen, if you see, for example, 25/150, it means you are attracting 25 foreign tourists and you need 150 to win via culture. The 150 number comes from the highest number of domestic tourists attracted by one of the AIs, so it means one of the AIs has 150 domestic tourists and every other AI has some smaller amount than that. If you are going for a culture victory and doing well, YOU probably attract more domestic tourists than everyone else, so the AI victory conditions might be something like 10/250 because you have 250 domestic tourists.
The tourist system does not seem to be a push/pull system like in CiV, but more of an aggregation. Basically the thread suggested that after generating a certain amount of tourism, you attract a foreign tourist. The number suggested this was 200 for standard speed I believe. In other words, if you get a great work with +4 tourism, and do not generate any other tourism, after 50 turns you will gain 1 foreign tourist who wants to see your sick relic or whatever. This system would suggest you cannot lose tourists and there are no tourism modifiers for individual civilizations like sharing religion, open borders, etc, since it implies the tourists you gained are essentially one from each civilization.
This would imply that domestic tourists are your "defense" in the sense that it increases the number of foreign tourists required to win the game. The thread, at least at the time I read it, did not go into this part much, but the logical corollary would be that you generate domestic tourists based on your culture.
Again, this is just me going from memory of this thread I am unable to find and isn't anything confirmed, but there was considerable effort made by a user whose name escapes me to figure out how exactly you gain a foreign tourist.