What would happen if I deleted everything in my Activity Monitor?
Not everything will die. Also some things will resurrect. launchd is the MacOS daemon that is responsible for sorting out what runs, and it also the parent of all other processes. It is configured to re-start some processes if they die (ie the Finder.) As an ordinary user you can’t kill processes not owned by you, so any root owned processes should be immune. If you could kill any of those you could cripple and bring to a halt most system operations. Killing the root launchd itself would usually cause the system to either reboot or drop to single user mode.
But depending upon the order you attack things you could stop a lot of the system from operating before you managed to bring things to the point where you couldn’t go any further. Of course one of the things in the activity monitor is itself.
If you kill the launchd instance for you, Your login will probably instantly end.
What do you mean “deleted”?
Quit / Force quit?
Remove from the display list of processes being monitors?
Find the 6$#@#!@ application or set of process files and delete them from your hard drive?
Something else?
My MacBook Pro has been getting warm recently, and the fan comes on. I didn’t even know it had a fan until a week or so ago. Turned out I’d somehow accidentally started (or improperly quit) Photoshop Elements, and it was using CPU. I quit it from the Activity Monitor. When I look at the Activity Monitor, it seems like there are a lot of things running, compared to what I vaguely remember seeing years ago. So I just want things running that are actually needed.
Would re-booting do this?
Yeah, reboot; that will leave you with only those things running that the OS always starts up + whatever things youv’e set as startup items in your Account prefs pane.
OK, I’ll do that later.
AFAIK, I haven’t set anything up as startup items.
In many ways all the various processes you see are just a structuring mechanism. Not all of them are even real processes. As features are added to the OS they are often implemented as a new process. Also, processes are removed from the OS as they come up with newer ways to implement things or they delete old features. In general you don’t need to worry unless you see a process that is actually using significant resources. Most of these processes are dormant unless they are needed. However you can find problems were certain processes spring to life and don’t seem to want to go back to sleep.
If you open up a terminal, you can use the man command to get information on most of the system related processes. Anything ending with the letter “d” is almost always a daemon, and has a specific job in running the system.
BTW, if you are adventurous, you can change things about the system that weren’t meant to be changed and see how it wrecks it.
I once messed around with my PATH system environment setting (trying to get some open-source software to run), and deleted the search paths to /bin and /sbin. I figured that I couldn’t really hurt anything doing this - it would only affect programs I launched through the Terminal, right?
Wrong.
It completely hosed my user account - I couldn’t launch any software, and was basically locked out. I had to hard-reset the machine and log in from a different account to fix it.
You modified it in your .bash_profile (or equivalent ie .profile .cshrc etc) file right? Noob mistake 
Yep.
I’m a bit more careful these days.