Moses and the entire generation that fled Egypt have passed away in the wilderness. They sinned against God and rejected Him and so they did not get to participate in the Promised Land. Instead their children and grandchildren get to come and possess the land. Joshua is Moses successor, he is basically Moses as general rather than Moses as tour guide. He is given the task to conquer the Promised Land and destroy the evil nations that live there.
Jericho was a massively walled city making conquest nearly impossible. It had two layers of walls around the town. It would take something special to be able to conquer the city and luckily for Israel they had God ready to fight for them.
This something special required to bring down the walls of Jericho involved some strange battle tactics. God commanded Joshua to march around the city each day for six days with the Ark of the Covenant while blowing ram horns. This was a psychological attack on Jericho. They had heard how Israel had escaped Egypt and feared what this God to do. Imagine living in Jericho and early in the morning hearing these ram horns going around the city. When you look over the walls you see this army marching around. Big walls or not that would be terrifying. On the seventh day they marched around seven times and then gave a long blast of the horn and everyone yelled and down came the walls. This way of taking down the walls showed that it was God who was fighting for the Israelites. It was not the power of men that would conquer the Promised Land but the power of God.
In this story we also see God's faithfulness. He saves Rahab because of her faith in God. Joshua had sent spies into Jericho to scope out the city. The king heard of this and tried to hunt down the spies to kill them but Rahab hid them from him. She knew that the God of Israel was powerful and would destroy Jericho so she sought His protection. Because she had risked her life protecting the spies, trusting that God would save her, God did save her and was faithful to Israel's promise to spare her. Rahab would live with Israel and was actually the mother of Boaz who married Ruth (book of Ruth). This family line eventually led to Jesus (Matthew 1)!
In this passage we also see another example of God's wrath. The evil done by the people of Jericho was deserving of complete destruction. The Israelites were ordered to destroy everything. These people were similar to the people during the time of Noah, all they did was evil and all they thought was evil so that there was none who followed God. Just as He used the flood to purge evil He used Israel to not only purge evil but to give the hope of God to the nations. This saving hope was experienced by Rahab.