There are a couple of different meanings and interpretations of what particles and particle animation software should do. In general, a particle is an object, or point in 3D space that is then controled by parameters mimicking real world physics. For example, take a point in 3D and apply motion to it as if it were under gravity. The result would be it would move downward increasing in speed. So you can use particles to simulate wind, exploding, melting and other things that move points in different ways.
Particles can also be objects or simple polygonal shapes rather than just points. In imagine, you can make a 'particle object' by modeling a normal object with faces. Then, using the particle requestor in the Detail Editor, specifiy that all faces/polygons of the object be replaced by cubes, spheres, tetrahedrons and so on...or even other objects. When you go to render the object, each face will be replaced with the particle you have chosen.
This means in Imagine, you can make objects created out of many 'particles' even though you don't use any particle type of motion. This is true since you could now take that object, and just animate it normally using standard key frame techniques and so on.
However, Imagine does have a new method for applying the motion control over these particle objects, (or even normal objects, since it will simply affect the faces). In the Action Editor you can now use the particle effect. This lets you specify how the particle/faces of the object will be affected over the course of an animation.
In general, particle systems allow one to simulate complex motion easily. They allow one to create many particle objects and to have their motion defined by mathmatical properties.