A hard bounce is an e-mail message that has been returned to you because the recipient's address is invalid (permanently undeliverable). A hard bounce might occur because the domain name doesn't exist, the recipient is unknown, the address is invalid (typos or changed address), the e-mail recipient's mail server has blocked your server, or there's some type of network problem on the recipient's end.
A soft bounce is an e-mail message that gets as far as the recipient's mail server (the address is recognized), but is bounced back before it reaches the recipient. A soft bounce might occur because the recipient's mailbox is full (over-quota), the server is down or overloaded, the message is too large or the user has abandoned the mailbox. Most e-mail service providers will attempt to deliver the e-mail regularly for a few days. If it is still undelivered, it becomes a hard bounce.