Is a synthesizer limited to the sounds it has onboard?
 

I am wanting to learn how to play piano (and hopefully make music in general). My dad gave me his old Kawai K11 (late 90's, has MIDI in/out). My question is that this thing has piano sounds obviously so I can learn my initial goal, but after that can it be used to play "new" sounds like if I have software on my computer and the synth is just providing key press signals?

Or can you upload new libraries to a synth? If you could point me to a good primer on how a synth and MIDI can be used that would be great.

Also since nothing seems to come with MIDI ports anymore (we used a sound card for this thing back in the day), is a USB to MIDI cable the most practical solution now-a-days?

Well, you should know the difference between a synthesizer and a keyboard, just in case you're in the market for either of them and buy the right thing for your needs.

A synthesizer is pretty much a keyboard, but you actually create the sounds yourself based on manipulation of oscillators, filters, LFOs, etc. This usually ends up giving you the palette of electronic sounds that you hear in electronic music. While there are typically "patches" that are saved snapshots of different settings, a synthesizer affords you more control over your sounds, because they are entirely made up of the manipulation of the controls.

A keyboard (in the context of containing grand pianos and other acoustic or non-synthesized instruments) simply contains actual recordings of the real thing, converted and optimized for manipulation on the keyboard. This is referred to as sample-based keyboards. You do have a degree of control in terms of manipulation, but the base sounds themselves are not synthesized, and come stock (or via add-on) in the keyboard.

Now, if you want to have access to sampled instruments like grand pianos, acoustic drums, etc., you'll want to get a keyboard, and not a synthesizer. Keyboards typically let you add to the library via add-in cards or downloading from an external source depending on the abilities of the keyboard.
That's basically buying a hardware keyboard that contains all its libraries in itself. That's the pricier option.

However, if you have (or prefer to obtain) software that has these sample-based libraries on them containing the sounds you like, you can simply buy a midi controller that will trigger the software. This is the least-expensive solution, and is often very much the same quality as an external-only hardware solution. Also note that this option is available for software synthesizers as well.
For this solution, you'd need:
A) A software program capable of producing the sounds you require, and accepts MIDI input. (all of them will accept MIDI)
B) A MIDI interface for your computer, whether it's built in or a PCI-E add-in card, or a USB to MIDI interface.
C) A MIDI keyboard.

With that setup, you simply connect everything accordingly, and you can record directly on your computer with either a simple recording software or a music program, and for sound, it will simply use whatever sound system you have connected to your PC.

The good thing is that with a MIDI keyboard/software combo, it's far easier to add libraries for the software. And the hardware keyboard can be used with any software you want.

Hope this helps.

You can hook it up to Reason or Fl Studio or Logic, and then find a sound you want to sample and play on the keyboard...

Yes, in this case the keyboard is a "midi controller" and the you would have to have what's called "soft synth" on your computer.

So when it comes to synthesizers you have a basic set of waveforms that are built in and you may think of these as the "engine" providing the basic sounds. You can "tune" this engine, but you cannot change the waveforms. You can modify and shape the basic sounds, but you cannot create new waveforms. This means you adjust the built in sounds, but you cannot for example create a new "organ" sound if there is no "organ" waveform to begin with.

A good book on synthesizers is this: Welsh's synthesizer cookbook, by Fred Welsh