VPS (Virtual Private Server) is a virtualization at the operating system level.
VDS (Virtual Dedicated Server) is a virtualization made at the hardware level.
In VPS, the server resources are used by a single operating system and this operating system shares the resources. The 2GB Ram assigned to the user is not actually dedicated to the user.
In VDS, each virtual server directly accesses hardware resources. If there is 2GB Ram assigned to the user, 2GB Ram is assigned to the user whether the user uses it or not.