I actually had the same questions when I started out with Relate/Community, so here goes:
Relate+ is not really a product but more a package that can install CMS, Community, the integration between the two and a bunch of example templates.
The easiest thing is to install the Relate+ package because if you only install Community you actually only get the administration interface and the API - no templates at all. So you would have to start from scratch. And since you probably often would integrate to the CMS you might as well install it to begin with.
You also ask if it is possible to install the community in the same site as the CMS. And when you install the Relate+ package you actually get a CMS site with CMS templates using community functionality, which I think is the way you would normally go: Developing CMS templates that uses the community API.
Finally: If you install the Relate+ package both the CMS and community data will be in the same database.
Hope this clears up some of your questions?
On the very techinical side of things, Relate+ is a bundling of CMS, Community (and Mail) with a template package built on top of these products included.
So it really depends on what you want and need, but to get started you probably want to install Relate+, that way you will have CMS and Community installed on a new site and also public community templates that you can modify/extend/use as inspiration. (You will get a site very similar to the public part of http://relate.episerver.com/, just to give you an idea.)
More info about Relate+ can be found at
I actually wrote a blog post about this a while back:
It is possible to upgrade CMS on a standard Relate+ 18.104.22.168 site.
However, it is probably of interest to you that an updated Relate+ package is now available for download:
It has updated versions of all the Relate+ components (CMS R2 SP2, Community 3.2 SP1, Mail 4.4 SP1 and the templates have been updated too).
Is it possible to integrate CMS and Community, as in the Relate package, but using only the stand-alone versions? If yes, where do you find some sort of documentation on this?
Actually with the new installer for EPiServer Community 3.2 SP 1 you can install Community onto an existing CMS installation. Then it's just to start using the API:s of the Community to build your site. I encourage you to look at the Relate+ templates for examples of how you could use the Community API.
What is the difference between the two products EPiServer Community and EPiServer Relate+? When you have installed EPiServer Relate+, both of these two products are visible in the Deployment Center. But which of the product should i install? Are there functions like communities built-in the Relate+ product?
Furthermore another question: will it be two different site/database for EPiServer CMS and EPiServer Community?, Is there a possibility to install the community in the same mainsite as EPiServer CMS? I dont really understand how the interaction between the two site will look like, and how to develop against the Relate product when the solution is the mainsite EPiServer CMS... Sry, for my newbiequestions, i have search for information about this on your site, but have not find anything.