If you have a rack mounted sampler (or any rack module) you need to connect some type of MIDI controller to trigger the sounds. Normally, this means a MIDI keyboard (although there are also guitar, percussion and wind controllers) Most MIDI keyboards have built in sounds (and often many other functions). however, some MIDI keyboards have no sounds and are designed solely to trigger modules.
The actual samplers can play sample disks or sounds can be loaded into a sampler from a hard drive. You can also connect a line level or mic level feed to the samplers and create your own samples.
Personally, I prefer to use samples on disk (or saved to hard drive). I've spent too much time creating my own samples and while it can be a good feeling to create your own samples - it can be very time consuming. I prefer to play more than program.
Regarding recommending a specific sampler - it depends on what you want it to do and how much you want to spend.
I have not used the Roland 9000 - but I heard one and was very impressed - the reviews and artists comments I've seen are all positive - I don't have any knowledge on the Yamaha.