On a Windows PC, which running processes are essential and which are junk or even potentially harmful?

When I go into Task Manager and look at the running processes, I notice a ton of active processes that I do not recognize and that I did not start. I realize that a lot of these are OS ones that are necessary to keep the system running and communicating with all sorts of devices. But how do I identify what each one of these is for and whether it is essential or not. Is there a source I can turn to for guidance and identification? For example, what are these for:
AbacastDistributedOnDemand.exe
wuauclt.exe
svchost.exe — mutiple copies of this one!
lsass.exe

Part 2 of this question is, once I identify one that is junk, how do I get rid of it? There is usually no “uninstall” for it, and it wouldn’t be found in Control Panel/Remove Program, either.