Rather than having the gravity pull the spheres into each other, they remain anchored. Instead, the shape of the sphere is tugged towards the other objects. The spheres then are warped depending on their proximity to other spheres. If this descriptive leaves you in the dark, take a look at this movie . It's all Metaballs and should give you a good idea of how they work.
BioSpheres are very useful for creating a complex object, and then fine tuning it with the brushes.
If you're interested in the basic mathematics of Metaballs, check out this article on Gamasutra . Another whole set of tools are the distorts . These tools affect the whole object, almost like a Photoshop filter for a 3D object. They include things like shearing, twisting, noise, and flattening. Another useful distort is the undo distort, which lets you variably undo your previous action.