I think that circle journals started becoming popular following an article in the US scrapbooking magazine, Creating Keepsakes.
I also found a great US website, Circle Journal (link over on the left), this is a retail site that sells kits but also has ideas which are really useful when beginning and you can also see journals done by others.
I have done several CJs, and am currently working on two from a couple of groups. For a circle journal you pick a theme and you can either create or purchase your CJ. I have made mine and some that I have been part of have been handmade or used shop bought albums like the 6x6" scrapbooking albums and 8x6" spiral bound journals. The choice of journal is entirely up to the creator!
The main thing to consider when determining your CJ is how many pages you need. Most CJs I have done have involved doing two pages. Inside, there has been an introduction page which gives the fellow CJ'ers the guide on what the creator would like you to do in their CJ (or not to do as that is just as important). There is also usually a "sign-in" page, where you sign and date when you have the CJ.
Themes - the list is really endless and the choice of theme is yours. One of my CJ themes has been "what's in your handbg" just out of curiousity I know my bag is full of stuff and every now and then needs to be turned upside down to see what is lurking inside, I usually come out richer from when I have just thrown change into my bag!
