[TheHobbit]

As said it all depends on what you are comparing it too and usoing in the signals path a way to explain this is thus...

Laymans terms

Using your eyes look at something familiar...a white painted wall.

Now close one eye...does it look any different? (mono - stereo)
Now with both eyes open look at the same wall whilst wearing a set of low end sunglasses (USB lets say)
Now look at the same wall with another a better set of sunglasses (your soundcard).
You are using the same set of eyes and looking at the same wall.
What has changed is the sunglasses and assuming they are different then you would have noticed a variation in 'tone' of the white wall and 'scope' with one or two eyes open.

Analogy explained :

Your eyes = the soundsource (waveform)
The wall = your speakers the end product of what you see
The sunglasses = what ever components are carrying the audio signal from the source to your speakers.

I really must stop visiting forums in the early morning ....