Procedural textures have been used for a long time ( by the standards of computer graphics that is ) in the 3D world where they are used to create effects such as fire, woodgrain, and marble. VTexture allows you to create such textures in Illustrator where they are rendered with paths as opposed to pixels.
In the Effects menu you will see a new entry VTexture : under it there are entries for Band, Fire, Marble, and Woodgrain which are the 4 types of texture supported. If you invoke this filter on artwork filled with a suitably-coloured gradient the texture will look much as described : play with the settings enough though and you can get wild-looking effects.
VTexture works only on path art that is filled with a gradient. You can get different effects for all textures except woodgrain by changing the angle/centre of the gradient and by making the gradient radial.
1) Start with a gradient-filled path. Using a radial gradient or changing the angle would give different results.
2) Select it and invoke Effects->VTexture->Marble...
3) Play with the settings.
4) Subsequent editing can be done in the usual way for Live Effects by double-clicking on the VTexture entry in the Appearance window : you may also adjust the gradient in any way desired.