someone who is easily controlled, offers very little resistance to what someone else wants to do, and backs down easily
"Carrie is such a pushover," said Mark. "She never stands up for what she believes in and is always letting people walk all over her."
Someone who does things for the best interest in others. They only want people to like them and will do anything for it. They will also do anything that others want just to make them happy.
"John doesn't usually do anything for anybody but himself."
"Yeah, he's just a pushover when it comes to pretty girls."