Info about permits needed to drive into or near certain cities...?

For a humorous response, the simple summary is that some cities have restricted-traffic areas. I used the link that you provided to check out one city that I am going to in a couple of months, and the map that's at the link you provided was not detailed enough for me to feel comfortable that I know where the zone in that city it.

What I do when I'm traveling is to google restrictions for each specific city that I'm driving to, and find a map that shows the restricted zone. Then I will either find a hotel outside of that zone, or find a parking garage outside of that zone and walk, with my luggage, to the hotel. I make the choice that I don't even want to deal with driving in the zone, even when the hotel says it can provide a pass or permit for their customers. That's obviously my choice.

I just ran into this restriction for Strasbourg when planning our trip for September. We're flying in and out of Frankfurt and renting our car there, so it won't have the required Crit'air sticker. We eventually decided to park the car just over the border in Kehl, Germany and take the tram into Strasbourg. They don't make it easy for tourists 😕

They don't make it easy for tourists

That's the whole idea. They want people taking trains and buses to reduce both congestion and carbon emissions.

Can anybody simplify / summarize this for us....?

Short answer: No. There are many different rules in different countries. But in general there are two categories:

• Car free zones, the most famous are the ZTLs in Italy but they exist in most countries. Cars are banned here, unless you have a special permission.
• Environmental zones, exist in many countries. Cars are allowed here if they meet the requirements. Usually shown by some kind of sticker. They can be rather large and cover entire cities, and being more strict in more central areas.

They don't make it easy for tourists

That's the point! We prefer to not have cars in our cities.