I can't figure out what is the difference between these two programs. Specifically, I'm interested in running a virtual server in the background, as a daemon (Linux host OS)
from the VMware website, Server is a hosted application, ie it runs inside a host OS. ESX runs on the baremetal.
VMware Server runs as an application on a normal OS Windows or Linux. ESXi installs it's own hypervisor as the base OS.
If you're asking about this, then it's likely that you should start out with VMware Server.
To elaborate on what origamigumby said: Being on bare metal allows the hosts on ESX to run at near real speeds, so you'll see if a big performance improvement over a standard VM. However, it is quite expensive, and if you are just using it for a test lab or the like, it probably isn't worth the money.
I have Windows Vista operating system as host operating system. I have VMWare 2 server installed on Windows Vista host operating system. If I decide to move to VMWare ESXi, do I loose my host operating system and all software inside host operating system ? Or can I try VMWare ESXi without loosing my host operating system ? Also I dont have multiple physical hardware - I just have one server (I dont have second laptop or desktop). Can I still try VMWare ESXi on my server ?