Noise Sculpture’s Get A Grip On Synthesis and Synthesizer Boot Camp tutorials offer a basic introduction to synthesis.

This video looks at modulation.

http://www.youtube.com/watch?v=95Euc8SdTyc

The next video looks at using filters to create sounds.

http://www.youtube.com/watch?v=oaP9hCtVXvE

This video looks at using envelopes as a modulation source.

http://www.youtube.com/watch?v=CS_MFw2Y9kU

This video looks at low frequency oscillators (LFOs) and how they can be used to modulate sounds.

You can find out more about the series at http://noisesculpture.com/bootcamp.html, and check out http://noisesculpture.com/books.html to read more about synthesis.