[Zythus]

Cornandbeans would probably give a more through definition and application, but heres mine.

Synthesizers, or Synths for short, is an instrument, like your piano, or your flute, or your trumpet, only that its electronic, with usually a digital interface. Think of it as your computer. Now, synths have many methods of input, including but not limited to : piano keyboards, guitar strings, etc. but predominately a piano keyboard. You can think of this method of input as your keyboard for your computer.
Notes played on your input is not sound, but electric signals or pulses. The Synth which your input is attached to uses the signals and subject them to a sound that you want to emulate, meaning, you can use a piano keyboard and press the C key. The output of the Synth (Speaker or headphones) can be the C key played by the sound of a flute, the kick of a drum, or even a sound you distorted to not sound like any specific instrument.
A Synthesizer is a piece of hardware, it is usually a bit more bulky than a piano with intricacies of knobs, levers, switches and maybe a touch screen. A softsynth on the other hand, is a piece of software you can run on your computer or as a VSTi plugin to your Digital Audio Workstation (DAW). FL studio has a few softsynths, such as Poizone, Toxic Biohazard, and Sytrus. These are just as good, if not better, than hardware synths due to their compact "size" and their versatility.


Now some things about using a Synthesizer.

Synths vary in GUI (graphic user interfaces). They might look different from each other, but they usually have the same assortment of knobs and such to create the same function. After inputting a note, the synth reads it and plays it out for you. It is your discretion how you want to enhance and change the note sound. With practice, you can change a child's voice into a weezy old man's.