Soft synths and samplers are made of softer plastic and other materials that are soft to the touch. There are also blow up rubber synths and of coarse samplers. The reason is weight. when you travel a lot carrying a hard synthesizer around can hurt you and others. A soft synth is therefore more adequate for this task. It can't hurt you or others if dropped or thrown. This also has to do with some musicians being rather violent and aggressive (just their nature). Obviously the aggressive types aren't allowed to use hard synths and samplers at all.

So here you go. ;-)

Okay. here's the real skinny.

Software synthesizers are just like hardware synthesizers but they only exist inside your computer. The only space they take up is hard disk space. They respond and sound just like hardware synths (in general that is) and are usually controlled by either a dedicated MIDi controller or any keyboard that has MIDI.
Editing of all the parameters is much easier since a software instrument (be it a sampler or a synth. Or both at the same time) is basically a program with it's own interface. All the parameters are available and can be stored and loaded as well. The advantage of software instruments is that they can load more samples. You can run as many of them as your computer's CPU can handle and most important of all they cost far less and simply deliver more for the money.
Some of us mad fanatics still use hardware instruments though. Mostly because each and every synth model has it's own unique character to the sound and so on.

Hope this helps. I've probably posted a ton of different threads and replyes so feel free to search the archives.

Later,

-ED-