Virtual manager is a application that connects to libvirtd in the back end. Think of it as a web browser or file manager for VMs.
Proxmox VE is an entire OS built for virtualization on dedicated servers. It also has support for clusters and live VM migrations between hosts. It is in essence a server OS designed to run in a data center (or homelab) of some kind. If is sort of equivalent to vSphere but they charge you per CPU socket for enterprise support and stability