NAND vs SDCARD (where is my memory used at?)

[m4f1050]

Ok, since I had this doubt I figured I'd start a new thread to keep other thread clean.

What I understand so far:

1. NAND (internal memory) is used as "hard drive" and to store OS (what about installed apps?)

2. SDCARD (external memory) is used as "external hard drive" same as internal memory

3. RAM (where OS and APPs load into)

Here are a few doubts I have....

When you boot your phone the OS gets loaded into RAM from the (in our case) SDCARD and when you turn it off this memory gets wiped out because it's volatile?

Is the NAND and RAM shared in some way? Is it partitioned in any way? I remember back in the WM2003 days you could slide and pick where your memory would be used at, but that changed a long time ago. I am guessing that got replaced by RAM/NAND combination?

On HTC website if you check phone specs it gives you 2 different memory sizes (I believe one is RAM the other is ROM) is NAND = ROM?

Is it called ROM but we can "write" to it? It's just a name for it that stores the OS ROM (or should I call it OS installation?) I'm sure there are a few of us that are confused because ROM IS READ ONLY MEMORY written once, never to be written again and again like a "hard drive"...

Examples:
TP2: ROM: 512 MB / RAM: 288 MB
Imagio: 512MB / 288MB (256MB Device + 32MB MSM), SD Card supported
HD2: ROM 1GB; RAM 576MB