Thanks but I'm still unclear. Hopefully this will make some sense...
Some Keyboards contain onboard sounds... synth sounds, pianos, etc. You can play those and hear them when not connected to a computer. That's essentially a Synth.
Most MIDI Synths can be setup to play their onboard sounds alone or through Cubase. They can usually also be used to send MIDI data (e.g. pressing "this" key, "this" hard for "this" long) - to Cubase. This can then trigger the sounds of a software based instrument hosted in Cubase.
There are also dedicated MIDI Controllers. They don't make any 'sound' independently. So they transmit MIDI information but NO sound. This enables you to only trigger sounds of a software based instrument in Cubase.
Which of the above two would you like?
Small MIDI Controllers can be purchased which are relatively cheap... 50-100 dollars for example.