Motionworks has a new quicktip, Effects A-Z: Box Blur, which leverages A Tale of Three Blurs, a Prolost article from 2006, to illustrate the key differences between Box Blur, Fast Blur, and Gaussian Blur.

There's a few more resources below to explore blurs further.

• If you want more on blurs, Chris and Trish Meyer have 17 movies on feature pro & cons of the Blur filter category in their Lynda.com series After Effects: Insight into Effects. Lynda.com has various free trial offers if you want to dig in.
• Motionworks previously posted on the Bilateral Blur filter, with guest host Eran Stern demonstrating Bilateral Blur.
• AE Help, of course, and posts tagged blur on AE Portal, where you can also survey previous tutorials in the A-Z series.
• Premiere actually has a Convolution Kernel effect with a 3x3 matrix if you miss KPT Convolver and want to roll your own blur, emboss, sharpen, etc.
• See FreshDV for Specular Highlights and Bokeh Primer(s). Andrew Kramer fanatics already know about the Lens Blur filter from the old tutorial 15. 2D Depth of Field. Virtual Cinematography in After Effects 7.0 is an excerpt on some blurs from the great book on AE by Mark Christiansen.
• Less exact than Motionworks' quicktip but branching to other territory is Blur 100: Introduction to Blurs, a video by Vfx Hauiku. He doesn't talk about Frischluft Lenscare, an AE filter used by many power users, but he does discuss related filters in Eyeon Fusion (see the AEtuts+ intro).