There are two main way to create particles: from geometry or from a point in space. There are tools on the Create Particles tab of the shelf that let you create particle systems by clicking the buttons. Clicking the Emitter button will allow you to select a point in space from which to emit particles, and the Source from Geometry button will allow you to choose a geometry object from which to emit particles.
In addition to these common ways to create particles, you can also split new particles off from existing particles, and emit new particles from the point where two or more particle streams cross with the Stream POP.
You can add particle-related attributes to geometry using the Point SOP, and the particles will inherit the attributes as they are born. This lets you define particle attributes in relation to geometry attributes.
How to use the shelf tools
For more information on how to emit particles, see the following help pages:
Emits multi-colored particles from a point, which simulates fireworks.
Emits particles from a point in all directions.
|Source from Geometry|
Emits particles from an object in all directions.
Splits particles in a particle system.