vst in general are effects such as delays, reverbs, flangers,
compressors, limiters, etc, anything to effect the audio coming from
a channel. vsti are instruments that you trigger notes and other
parameters with, from a midi input or midi track in your sequencer.
So essential the so called sampler in FL is a vsti, or the 3 Osc
joint is a vsti (strangely enough in FL they are called "generators")
and those are vsti native ONLY TO FL and can not be "directly"
hosted by other sequencers such as cubase, cakewalk, ableton, logic
or whatever. And the so called fruity delay and fruity delay and
so forth are vst native only to FL. Then when you go under the
channels menu and the more selection, it will show you all the
vsti and vst including THIRD PARTY joints that you may have installed
into a vst folder of your choosing. So you can use another type
of "generator" (vsti) in place of 3 Osc or whatever in fruity,
that may have different sounds you want, or you can use another type
of compressor or limiter instead of the FL compress, Waves L2
for example, or any type of free vst's you can find at
http://www.kvraudio.com (they review commercial ones too)

I'll talk about automation of vst and vsti parameters later.
Peace.