What exactly is "redistribution?"

[Zach Leatherman]

According to the BSD license, redistribution and use are permitted
given three stipulations are true.  However, the first two
stipulations only refer to redistribution and none of the three refer
to use.

http://www.opensource.org/licenses/bsd-license.php

Last week I found some of Yahoo's CSS source code being used on
Google's Personalized Homepage.

http://www.zachleat.com/web/2007/04/05/google-using-yui-grids-css/

Some have argued that Google doesn't need to attribute the source to
Yahoo, because the BSD license with which Yahoo has released the code
only restricts redistribution and not basic use.  This would seem to
make the most sense.

My question is, what is the difference between distribution and use in
a web environment?  Because the files are hosted in their application,
they are not really redistributing the code as a whole, just 4 or 5
lines of it.  Just for my curiosity...

Thanks, Zach