In it's simplest form, that's right yeah, 1 ROM at a time, then you can flash others when you want to change. There are other steps involved (backups & wipes etc)

Normally your device won't boot into clockwork by itself, there's a special button combination you need to do in order for it to boot into the recovery image. This will differ dependant on handset. On mine, it's volume down whilst powering on.

Which phone do you own? I'll move your thread to the relevant forum and users of the device can give more specific assistance.

 

I do know one phone has a custom kernel that does have dual boot support

but ya, El Presidente is correct.

 

Then you're in the wrong forum, I'll move this.

That does however throw a small spanner in the works as you're new. To install ClockworkMod so that you can boot directly into it, you have to use NVFlash, ROM Manager will only fake flash it (need to use RM to enter CWM)

If you have NVFlashed, then hold both power and vol down until the blue LG/orange Android appears, then release.

 

yes.. only 1 OS/ROM can be on the phone at a time. you can have as may files that you want on the SD card (space permitting). a file is a file is a file is a file.

it should not loose the flashed rom.. when rebooting..
i am going to guess... your phone does not have a full root.