I'll have a go, and I'm sure that my colleagues on the other side in Saskatchewan may want to help me out as well.
Selective breeding is the very basic level of plant breeding. If you were doing some sunflower breeding in your back garden, you would be crossing different types of sunflowers and then from the seed produced from those crosses you would be looking for improved sunflowers: be they higher yielding, or different colours, and all those sorts of thing.
At a more sophisticated level you can start doing that analysis using genomics and micro-assisted breeding and all sorts of other sophisticated laboratory-based techniques to understand what the variation is in the first place, and to understand how to select the traits that you're trying to extract from breeding. That's selective breeding.
Plant biotechnology short-circuits that for very specific traits, such as insect resistance, herbicide tolerance for weed control, drought tolerance, and things like that. But you have to think of the two together, if you're thinking about plant biotechnology in general.
I'm not sure if I've helped you there.