The Question and Answer section for The Grapes of Wrath is a great resource to ask questions, find answers, and discuss the novel.
Hooverville is an independent camp of migrant workers the Joads join when they arrive in California. The camp is filthy and the residents are hopeless. There is no work to be had near the camp, and the local police have burned down the camp...
I think that Noah's limited mental capability made it hard to stay with the family. The family already had enough burdens without having to worry about Noah. I also think that Noah knew he could not take the stress of the ordeal.
Generally, they were searching for work in California. They were lured by the promise of picking fruit on the farms.