The blur variation generates a filled-in circle. Unlike most variations, which transform the plane by mapping input points to output points, blur completely ignores the input. The variations blur_circle, circleblur, and sineblur also generate filled-in circles. They differ in how the density is distributed across the disk. Blur has high density in the center, gradually decreasing towards the edge.
Think of tags as like a book index where you find words and which pages they are on, the same principle applies here. Although it's taken a stage further so it might not be the word, but the subject, so for example if you're looking for things to do with fractal gradients for example. A tag may bring back posts with gradient files, as well as posts that discuss gradients, or tutorials about them.