I have the software sampler
Sampletank 2. It comes with more instrument sounds than even Quincy Jones and Queen could use in a lifetime. These sounds are actual samples of actual instruments, rather than synthesized versions of instrument sounds that didn't really sound accurate. When you play the sounds on Sampletank you're getting those sampled sounds. Now, to me, they
are VSTis, which I understand to be virtual instruments. I have other VSTis such as
Celtic Instruments,
Danny Thompson double bass and
Zero G harmonica which I have loaded into Sampletank. They contain single notes so you can play whatever melodies or notes you want to play. They also contain riffs and melodies that I suppose you can work into whatever song you're recording, if you are that way inclined (I tend not to be). On the other hand I also have Cubase 5 loaded into my recording computer (I used to have Cubase SE) but because I use a standalone DAW, I use Cubase to house my other VSTis. So loaded into Cubase is
First call horns,
MTRON (a mellotron),
Lounge Lizard (electric piano),
B4 (Hammond organ),
Miroslav Philharmonik (orchestral instruments) and
Swar systems (Indian instruments). These give me a range of colours to add to my recordings. They vary in quality, but again, to me they are VSTis (virtual instruments), though they are referred to as samples and sometimes sample sound libraries.
But some DAWs come with VSTis. Cubase 5 came with hundreds of sounds from Halion. But I personally thought they were all fairly lame except for some of the synthesizer and sound effects sounds. So I don't use them. It's hard to say "this sound is better than that sound" because it's really so much down to personal taste. I know what I like. For instance, I used to use the violin and viola sounds in Sampletank 2 until I got Celtic Instruments and Miroslav philharmonik. They're so much better that I'd never use the Sampletank ones on their own again. There's so much variation.