Virtual Machines: Newbie / novice questions

[rhkramer@gmail.com]

I've never used a VM but I wanted to ask some newbie questions about them.  

Currently, I don't see much use for one--I haven't had to run Windows in a 
very long time, which is a good thing--I used to have to occasionally run 
Windows to do something that, at the time, I couldn't do in Linux--oh, I guess 
I still have to update my Garmin maps in Windows, which I do very rarely--only 
before a long road trip or when I discover some discrepancies based on new 
road construction.

I often have parts to build all or most of my next machine around, and, 
sometimes (as now) that machine is built and has the next generation of 
software on it for "testing and tuning" (as now, I have a 2nd machine running 
Jessie while my everyday "user" is running Wheezy.

I have some assumptions / guesses about the answers to these questions, and I 
could google for answers, but maybe somebody will indulge me with answers.

   * Is it right to consider that there is somehow a base OS and the other VMs 
somehow are set up on top of it--you boot into that base OS first, and then 
have the ability to switch to another VM.  OK, I'm thinking that is not the 
case--I think I've heard of things like a hypervisor (??), and now I'm 
guessing that the machine boots first into the hypervisor and then from there 
boot into any of one or more VMS, one of which would contain what I consider 
my main or base OS?

   * I know (or I'm 99.9% sure) that the software for a VM is on separate 
partitions (except maybe some are or could be shared between the VM and the 
"base" machine, such as for my documents and such (things that I'd want to be 
accessible from any of the VMs).  

   * Can I keep a VM in a running state for instant switching to it?  (I 
presume that in that case, it will be using RAM, on the other hand, it could 
be in a non-running (either non-booted or suspended-to-disk) and not be using 
RAM?

   * I presume that booting a VM takes  about the same amount of time, maybe a 
little longer than booting the base machine /  OS (well, unless the OS in the 
VM is significantly "smaller" in terms of footprint and services started at 
boot)?

   * Can I C&P between (applicatons in) different VMs and/or the base OS?

Thanks!