Second tier PPC search advertising networks are other search ad vendors than Google and Yahoo/Bing Network (for US market).
Some clicks are not search related but result from contextual ads.
An example of 2nd tier network:
Second tier PPC networks count for less than 10% of search queries on US market. Most known 2nd tier networks are:
The benefits for using 2nd tier networks are mainly the potential to extend the reach beyond Google and Bing/Yahoo for same keywords and to have lower cost per click as there is less competition.
The potential downsides and pitfalls of second tier PPC networks are:
return on time and efforts (spending time for few clicks)
Some 2nd tier PPC networks deliver several billions of search ads every month and offer many targeting capabilities on their platforms.
Networks whith less volume and more limited features are sometimes called 3nd tier networks.
How a network distribute search ads on search partner sites: