The best one line description I have seen...

"The Microsoft Office SharePoint Web application builder"

That pretty much sums up what I use SPD for.

As seen on the new Office Online site
http://office.microsoft.com/en-au/sharepointdesigner/FX100487631033.aspx?ofcresset=1

Until now, this content has only been available from computers that had a beta version of Office 2007 installed.