It may sound dumb but what is a rom I'm thinking its a ''theme'' ?
Not dumb at all.
I'm just trying to think of a way to explain without adding to complexity.
A ROM (typically) is the Operating System (in Linux, the kernel component) + the user-space components of everything on the Droid, with the possible exception of the "radio" (which is usually burned separately) and the bootloader, which consists of some 'hard-code', and some other firmware which can also be changed (like clockworkmod recovery or sprecovery).
So it is nearly everything. For example: Wifi consists of a) hardware, b) device driver or module in the kernel, and c) user-space routines to access it that will show up in the settings of the droid. That's the way many things work that to you, appear as "apps".
Those are all 'modified' by applying a ROM. They aren't all really changed, although the "framework' that makes up a theme maybe is changed, and other "operating" tweaks are made, code can be changed, etc.
Kernels can be recompiled to have new modules that allow tethering, mounting other filesystem types, and allow over/under clocking/volting, but they will usually require some user-space code to actually do the run-time tweaks.
Hope that helps.
If you think of the droid as a bunch of layers, they might be:
1) hardware
2) bootloader to load the OS
3) kernel
3.5) services (background tasks that manage e.g. networking, display)
4) user-space routines and apps
5) theme (if there is any special one), but Sense or Blur are examples of this layer of things.
Another example of what a ROM is would be what an OS is to a PC or Laptop: So if you installed Windows or Linux on a PC, it would be the corollary of applying a Rom to the droid, though most droid flavors are close to the original (so Android OS).