In looking for more musical instruments to add to your sound, there's no doubt that you've come across a synthesizer. The dedicated musical recorder already knows exactly how a synthesizer works, but for the unitiated, it takes a little bit of explaining.
The term "synthesizer" is essentially a generic term for a keyboard and computer set that will generate artificial noises at specific pitches. In other words, if a computer was a musical instrument, it would be a synthesizer. Listening to the top hits of today's music, you'll hear the work of plenty of synthesizer effects, across a wide range of musical genres. You can find a relatively typical synthesizer from Korg by viewing this video.
Because the synthesizer is organized in the form of a piano keyboard, you only really have to know how to play the piano in order to play a synthesizer - or at least know the notes on the piano. Many effects on the synthesizer will play themselves out if you simply hold in one of the keys, allowing the electronics to do their work.
What is a good synthesizer good for? It depends on what you need, but you can find some good uses for synthesizers in the following instances:
Recording music for an album.
Synthesizers don't play as well live as they sound on a computer with direct access to your ears via headphones, so they sound clearer and more vivid when they're recorded for a CD or an MP3 file. Many of today's producers utilize synthesizers to handle essentially all of the recording work, since it means they don't have to spend money acquiring other instruments that used to be used in recording.
As mentioned, synthesizers aren't ideal for live performance, but they do provide a versatility that other instruments don't have. Piano players who can't lug a piano around wherever they go can find that bringing a synthesizer with them to performances works as a great way to bring their talents around to their shows.
Is a synthesizer for you? Only you can answer that. If you're already interested in working keyboards and recording your music, a synthesizer is a must-have. For live music, it might be considered a "pleasant bonus." If you do decide to buy a synthesizer, be sure to do plenty of research into individual features to find out what you might enjoy the most.