No. There has been lot of gossip in the media about the introduction of a special permit to limit the number of tourists in the Cinque Terre. Yes, there have been talks about it, but this system has never been developed, so you don't need to register or get a special Cinque Terre pass.
There are two different Cinque Terre cards. With the Cinque Terre Trekking Card you can hike on the famous Sentiero Azzurro, or Blue path. You only have to buy the Cinque Terre Card if you would like to hike the Blue path, the coastal trail connecting the five villages. Please note that the Via dell'Amore, the first section of the Blue path between Riomaggiore and Manarola has been closed for many years, and only part of it has been reopened on the Manarola end. The second part of the path, the Manarola-Corniglia trail is also closed. So you can only hike from Corniglia to Vernazza, and from Vernazza to Monterosso on this path with the card. As that's the hike most daytrippers do, it can get very crowded, while there are hundreds of kms of free hiking trails in the area, with better views and less people. So for all the other hiking trails, you don't need the Cinque Terre Card. With the Cinque Terre Train Card you can also hike the Blue path, plus it includes unlimited train travel between Levanto and La Spezia, with stops at every Cinque Terre village. You can read all about Cinque Terre trains here.
Well, you have to do the math here. The one day Cinque Terre Train Card for an adult costs 18.20 EUR between March and November. A single ticket for the Cinque Terre Express costs 5 EUR. So if you take the train at least 4 times that day, it's worth buying the Cinque Terre Train Card. Or it's also worth getting the card if you're planning to hike the Blue path and take the train 3 times the same day. It all depends on your travel plans. If you have the card it's also more comfortable as you don't have to worry about buying the single ticket for every train ride, not to mention the queues at the ticket machines. Let's not forget that with the card you can also use the small buses that run in the national park. These buses can take you up to some of the best viewpoints of the Cinque Terre, if you wouldn't like to hike up there.
It's a day card, so it is valid for one day, meaning you validate it during the day, and it's valid until midnight. There are 2-day and 3-day versions as well.
You can buy it at the train stations or at the tourist information points in the Cinque Terre villages, Levanto and La Spezia.
Yes, you can buy the Cinque Terre Card online on the website of the Cinque Terre National Park. It's a good idea to buy your card online, this way you can avoid the queues at the ticket offices.
Please don't forget to validate your Cinque Terre Card at the green machines at the railway station, and write your name on the card. In case of a family card one name is sufficient. The controller might ask for your ID, so make sure you always have it with you. Children under the age of 4 can travel on the Cinque Terre Express and can use the other services of the National Park for free as long as they are accompanied by an adult holding a valid Cinque Terre Card. In case of orange or red weather alert the sale of the Cinque Terre card will be suspended. The sale of the trekking card will also be suspended if the Corniglia-Vernazza-Monterosso trail is closed for some reason.