VMware Converter and vCenter Converter are very similar.
VMware Converter is a standalone product that you can install on any Windows System, then run through the wizard and convert the local, or a remote, physical or virtual machine to a virtual machine hosted on an ESX Server. I really only use it in the few isolated networks I have that have a single ESX Server and no vCenter Server.
I primarily use vCenter Converter. This is almost exactly the same as VMware Converter, except that it's a plugin to vCenter. After you install it on the vCenter Server, you see it as a plugin on any client you run the vSphere Client from. Once installed on the client, it allows you to right-click any host, cluster, datacenter, etc and select "Import Machine". It presents you with the same wizard as seen in VMware Converter, but shows the progress as a Task in the Task Pane at the bottom of the vSphere Client window.
I've tried a few P2V's using VMware Converter 4.3, and it seems terribly slow. It takes over 4 hours for some servers. The vCenter Converter plugin is version 4.2, and when I use that to convert the same source machine it does it in less than half the time. VMware Converter 4.0 didn't have this issue, but I do see it with 4.3.
Hopefully this makes some sense and explains what they are.