Many operators let you choose geometry to interact with. For example, in the 
Collision POP, you choose geometry that particles can collide with.
There are two ways to reference geometry operators (SOPs) in particle operators:
Create an object and then click the
Source from Geometry button on the Create Particles tab of the shelf. The particle system will attach itself to the object you have selected.
In the
Source particle node, set Geometry Source to Use Parameter Values. In the SOP field, enter the path to a geometry (SOP) node.
Using the shelf is usually more convenient and flexible since you do not have to enter explicit paths and you can switch the geometry input easily.
Tip
Turn on the 
template flag on a particle node that references geometry to see the referenced geometry as a guide.