There's an interesting discussion about the musicality of arrangers at
http://www.korgforums.com/forum/phpBB2/viewtopic.php?t=105509. There are some heated opinions involved.
A workstation is a fully programmable keyboard which includes serious sequencing capabilities. It's multitimbral (can play multiple sounds simultaneously) and gives a great deal of control to the musician, assuming they plan ahead.
An arranger is a keyboard with built in rhythm and accompanying parts, which usually includes some sort of chord recognition. It's generally a better instrument for winging it, although the higher tier keyboards include some programmability and the ability to import additional sounds and accompanying parts. Lower end arrangers include built in speakers, while upper end arrangers have add-in speakers.
As computers and keyboards become more sophisticated, there is more and more overlap between these categories. However, one way to determine if a keyboard is an arranger is if there's a dedicated set of accompiment buttons. Top end arrangers are reviewed in
http://www.keyboardmag.com/gear/1183/top-end-arrangers-reviewed-korg-pa4x-and-yamaha-tyros5/55871, which also begins by addressing a bit of the differences.
A ROMpler is a keyboard which is primarily (or completely) based on playing back samples that were loaded into the keyboard. In some cases, additional sounds can be loaded, but that would be a higher tier instrument. A ROMpler would not include synthesis capabilities. This does not mean that a ROMpler is a lower tier instrument. All instruments are, by their very nature, limited in one way or another. My DX7, which was probably the most popular synthesizer in the world for a long time, created sounds by modulating sine waves with other sine waves (FM synthesis). It didn't have filters. It couldn't play any sort of sampled sounds. Prior to that, I used my father's synthesizer, which used subtractive synthesis, in which the keyboard produced a voltage which drove an oscillator. A filter then modified the sound of the oscillator by only letting certain sounds through. This is the style of synthesizer used by classic Yes, Emerson Lake & Palmer, Genesis and almost all rock from the 1970s. My Korg Kronos has nine different methods of generating sounds, and one of them would probably be classified as a ROMpler, since it plays back (probably modified versions) of sounds which were previously sampled. Other sound generation within the Kronos can be classified as FM synthesis and subtractive synthesis. But it, too, can't use all potential forms of sound generation.
And I just realized that sampled may be a term you don't understand, either. A sample is a recording of a sound. A sampler can play back that sound, transpose it up or down based on the key being pressed, loop certain portions of the sound so that it lasts longer than the recording, and modify the sample with other samples, filters, effects, and other modifications.
Hopefully this will help. But if it doesn't, feel free to ask more questions. There are people on this forum who are more knowledgeable, and more importantly can explain things better, than I.
