"Dia is specifically billed as a "diagram editor", which is of course where the name comes from. Though I've never used Visio, I have heard Dia compared to it. The main features of Dia are a deeply-integrate system of symbol libraries and a small set of primitive graphics tools which provide text, boxes, ellipses, polylines, and bezier curves. In principle, you can create vector graphics "from scratch" in Dia, but the interface favors using the built-in symbols and simply connecting them using "zig-zag lines" (or "connectors").
"The coolest thing about this is the system of anchors which allows connector lines to stay attached to the anchor points on symbols even as you move the symbols around. There is even an "auto-routing" mode which can be used with the connectors. This means that Dia is an excellent tool for "sketching" a diagram before you are fully sure how you want to arrange it on the page (a very useful feature, even though I don't really need it for this CUPS filter diagram project, because I'm working from an original prototype of the drawing)."
