Reference machine - A system (can be virtual or physical) you use to create a base image. The usual situation is you freshly install Windows on a physical system or new VM and then install the software you'll think you need. If it's a physical machine, you can use VMWare tools to "p2v" a physical machine into an image.
Image - Technically this would only refer to the file containing the virtual hard disk, but it can also mean all the files that together make what VMware needs to run a virtual machine. This includes a couple files containing configuration information, a file containing the virtual BIOS's NVRAM, and files containing all the data on virtual disks (.vmdks)
Base Image - An image you keep around, but don't use. You copy it, and then use the copy when you want to deploy it. This way you have a prepared virtual machine "ready to go" to rapidly deploy to multiple people or use for multiple different uses.
Snapshot - When you make a snapshot, you tell VMware to create files that save the state of your VM. You can then roll back to the state of the VM when you took the snapshot. So you could do this to restore a system to a known working point (losing all data, of course) if needed. I believe, but am not sure, that VMWare will only save the differences between your base image and snapshot, reducing the amount of disk space used.