The 
Follow POP makes particles follow a leader or
leaders.
You can optionally specify the leaders to follow using a bunch of “leader particles”: either a group within the input stream (set with the Leader group parameter) or the particles from a second input stream (connect a particle stream to the node’s second input).
The “leader” can be a computed point based on the leader particles (the spatial center of the leader particles, the center of mass, the center of density, or individual particles).
Each particle in the leader stream can lead individual particles from
the follower stream. Houdini can automatically assign followers to
leaders, or you can set a follow attribute on particles to determine
which leader they will follow. Use the 
Property POP
to set the follow-related attributes on particles.
How to use Follow
Start with two particle streams.
Use the
group node to put one particle stream into a group called leaders. Use another group node to put the other stream into a group calledfollowers.Use a
collect node to collect the two particle streams.
Click the
Follow button on the Drive Particles tab of the shelf.
On the Leader tab, set the Leader Group to
leaders.
Note
You must customized the parameters of the particle and follow nodes in the parameter editor. This tool is quite advanced and simply putting down the nodes will not automatically generate the desired result.
For specific parameter help see the Follow node help.