That kind of effect is sometimes described as 'particles'. Effects like it on a large scale are often created using particle generators in rendering software like those in Adobe After Effects, 3D software or in HTML5.
In a logo, however, it's more likely to be done by hand in regular design software, for fine control - but the term is sometimes still used because the final effects are often similar.
Here's a fun interactive particle version of the Google logo from Google Labs nicely demonstrating the overlap between hand-controlled design and live, software-driven dynamic particles. Each particle / blob / dot / circle's position has been carefully set by some designer, and they've also been programmed to have particular behaviours based on what the mouse pointer is doing.