"They have left the straight way and wandered off to follow the way of Balaam son of Beor, who loved the wages of wickedness. But he was rebuked for his wrongdoing by a donkey - a beast without speech - who spoke with a man's voice and restrained the prophet's madness." NIV translation
Peter reminds us of Numbers 22 and the story of Balaam's donkey. Balaam was a prophet who was tempted to go against the will of God. The Lord had to stop him from going against His people and the only way that He could get Balaam to listen was by making his donkey speak to him. Even as Christians, we can get pulled out of the will of God for our lives. That is why it is so important to have leaders that we can listen to and trust. God can use a donkey but He prefers to use people.
"These men are springs without water and mists driven by a storm. Blackest darkness is reserved for them." NIV translation
As people, we require water to live and in both of these cases there is not enough water and so their ways will also end in death. Peter could have been speaking to the church today as we see this more and more now. There are many that get up in front of large groups of people to teach but they don't actually hand out the Word of God. There are others that may draw a big crowd by telling them things that they want to hear and portraying God as some kind of piggy bank where we can get whatever we want. In both cases, they are not clearly proclaiming the Word of God and so they do not have life in their words.
"For they mouth empty, boastful words and, by appealing to the lustful desires of sinful human nature, they entice people who are just escaping from those who live in error." NIV translation
The words of these false prophets glorify themselves or "their ministry". They tell the people what the flesh wants to hear and deceive many new believers. These new Christians have not had sufficient time to grow and so are not prepared to face these men. They are like the birds described by Jesus in the Parable of the Sower (Matthew 13).
"They promise them freedom, while they themselves are slaves of depravity - for a man is a slave to whatever has mastered him." NIV translation
These false teachers are trying to tell the people about something that they don't even have. It is like going to a prison to ask a prisoner what the weather is like outside. (They don't know so they cannot tell you.)