The original Traveler Notebook has been around since the early 1950's. Called the Midori Traveler's Notebook, it is an import from Japan and actually made in Thailand. Fans of the traveler notebook style call anything not the Midori a 
"Fauxdori." There are a lot of Faux's out there and many have rightly earned their own names and own fan following because of the high quality of workmanship that goes in to each one. Almost all of the Faux's are made by hand, one at a time, and often customized by the creators. I don't actually know of ANY genuine leather Faux's that are not made by hand, hence the cost of the cover which, depending on size, can run anywhere between $30 and $100 and higher depending on how much customization you want to your cover.
What sets 'Dori's apart from other types of planners such as Filofax, Erin Condren, Cocoa Daisy, etc is that the 'Dori is very versatile in how you choose to use it and set it up. There are literally no limitations when it comes to how to use it. The basic principle behind the 'Dori is this: it comes with elastic bands down the spine that hold your inserts inside. The inserts can be changed, moved around, and you can make your own inserts. Midori original fans call them refills....
The number of elastic bands down the spine vary with style and customization. The number of bands does not limit the number of inserts you have in your Dori as there are ways to hold more inserts...see TheResetGirl on YouTube for more on this.
There is a whole world of artists making inserts, dashboards, folders, pockets, stickies, clips, and more so that you can make your 'Dori exactly the way you want it for your exact needs. Many 'Dori fans have more than one, I myself have three and use all of them. Which brings me to size. The original Midori is considered the 'regular' size that other 'Dori's are compared to. There are also larger 'Dori's and smaller 'Dori's, clear down to the cover that is the size of a standard passport. Many 'Dori fans use this size for a wallet. You can look here for exact measurements of the cover and the inserts for all the different sizes. This chart is from MorganLeFaesTrinkets blog, which also gives new users a host of information on setting up notebooks.
I would go so far as to say that 'Dori's have a cult following and people who use them are extremely passionate about them and protective of the artists who create them. There are a multitude of blogs, Facebook groups, Instagram Profiles all centered around the 'Dori. I have a lot of links on my blogroll on the right side of my blog that will link you to cover artists, supplies, and blogs. If you have questions feel free to post them. If you are a longtime fan, please share this post with those who are contemplating their first 'Dori.