What is the fifth wave and how does it fit into the larger scheme?
The fifth wave is Sammy and the other child soldiers who are being trained at Camp Haven (367). Both Ringer, Ben, and Cassie realize this vital information. This fits into the grander plan of the Others because it will allow them to pick off the remaining humans without actually having to hunt them down themselves. They have found the perfect weapon against the remaining humans: their own children. This comes after the first dour waves, which were an electromagnetic pulse, tsunamis, the red plague, and the Silencers.
What are the parallels between Ben and Cassie?
There are several parallels between Ben and Cassie, starting with the birth of their siblings. Both love and care deeply for their younger siblings and would do anything for them. They are also both motivated by the memory of their siblings in order to do something. Ben is motivated by the death of his sister to save Sammy, and Cassie is motivated by Sammy's absence to continue fighting to live, so that she can eventually rescue him.
Compare and contrast Ben Parish and Evan Walker.
Obviously, both are boys who Cassie is attracted to. Both also agree to help her save Sammy. Evan is motivated by the love he has for Cassie. He has been transformed from a human possessed by an Other as a host, to something more akin to a human again. This is all because of Cassie's love. Ben, on the other hand, does not reciprocate Cassie's crush and is motivated to help Sammy because of his sister. He also shares a flirtation with Ringer, and is motivated to become a better shooter in order to make sure that she wasn't transferred into Squad 53 for nothing, since her previous squad was in second place.
Compare and contrast Cassie's perception of the Others with what Evan tells her.
Cassie sees the Others as highly intelligent and very dangerous. She has seen them kill on multiple occasions and doesn't hesitate to kill the Crucifix Soldier when he is only a potential threat. She knows that she can take no chances with the Others watching. She often likens the Others to humans and humans to cockroaches in order to make the technological and intellectual gulf more real to the reader. Cassie sees the Others as a monolithic and all-powerful organization. Evan, however, reveals that this is not always the case. Yes, the Others have a grand plan at Wright-Patterson to make it a soldier and death camp, and Vosch is their leader, but the Others are also divided amongst themselves. Some view the humans as a disease that needs to be wiped out, while others don't believe that.
What is the purpose of Camp Haven?
Camp Haven acts as both a death camp for adults and as a training camp for children under 18. They are trained to become soldiers and indoctrinated into the belief system of the Others without realizing it. They will be used to fight and kill off the remaining humans, mistaking them for Others because of the glowing green their heads will emanate through special goggles. These goggles pick up humans who do not have implants in them and glow green. Camp Haven is the headquarters of the Others, and is lead by their overall commander, Vosch.