I have the same questions, I have done some searching and have yet to understand what the difference between rooted and unrooted and Roms. I am also new to this and would like as much info as possible.
Hello,
A ROM, I believe, is an operating system install package similar to a disk that may allow you to install a different version of Windows or OSX. When you restart your phone while holding down a special series of buttons, the phone unzips and installs the contents of that ROM onto your phone's permanent memory. This is just like upgrading to Windows 7 from Vista. The operating system completely overwrites and replaces the original one. This process is called 'flashing.' I believe they are probably called ROMs because they 'flash' overwrite the Read Only Memory inside your phone. 'Flashing ROM' is what you do on your computer when you upgrade to a new version of BIOS.
Root vs Unrooted
This is a term that derives from Unix bases systems like Linux and the Mac operating systems. Rooted means that you have hacked the operating system so you have root, or superuser level access over the operating system. Android is based on a linux kernel. Root lets you sign in as 'su,' at which point you have full access to every aspect of the operating system.
The Windows equivalent would be signing onto your computer from the main Administrator account versus signing on from the Guest account. Both will let you USE most of the software on the system, but only the Administrator level lets you change certain functions, settings, or install certain types of programs.
A rooted ROM, or version of Android, is one which has been hacked to allow you, the user, to have that full, absolute control over every aspect. This is very useful to very advanced users who wish to micro control every aspect of their hardware. It also allows you to uninstall some of the default software your provider has built into the system.
An unrooted version simply means that while it upgrades to 2.1, in the case of the Eris, it does not ALSO give you that absolute control over every possible detail. Simply put, you cannot access 'su' or superuser login. It is un-rooted, meaning it has not been cracked to allow access to the root level privileges.
If you are confused by things, don't worry - everyone except the developers and the really motivated, dedicated Eris superstar users is, too. There are a few different versions floating around.
You would want to read carefully, figure out exactly which version you're looking at, and check to see what others who have used that version are experiencing.
It seems like a lot of the contributers to this forum are really knowledgeable and friendly. Asking them exactly which versions they've used and what bugs or problems they've encountered might help you decide which version is right for you. Also, there are other android forums out there. I get really confused trying to figure out whether they are using the same ROMs or have different devs putting out different versions than the ones I see here at android central.
I'm personally waiting for the official upgrade, because I've only had my phone a few weeks, and it's still new enough to me that I'm content with it as things are. I mean, it's not like I've been waiting since November like most of the people here in the forums who have already upgraded. And I don't use navigation software, anyway, so the one or two features I'd gain in the short term just aren't that important to me.
I wish I could afford a second phone just to experiment on. If so, I'd try them all.