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.
Variations based on polynomial Julia sets. For escape time fractals, the formula is z → z^n + c. For flames, the mapping is reversed and the “+ c” part is done by the affine transform, so these variations basically take the root of the input taken as a complex number, returning one of the results at random.
Although “linear” seems a strange name for a non-linear variation, it’s a really important one. The name makes sense in the context that it means not to use a non-linear variation, just the affine mappings that are part of every transform. When a new transform is created, it is assigned a variation of linear3D with an amount of 1, which just maps the input to the output without any change.
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.