Temporary bans last for 10 minutes (usually) and can occur for various reasons ranging from an unsuccessful connection attempt (connection was broken while sending a command for instance) to you being kicked from the hub (most likely to happen).

Permanent bans are entered manually or scripted, and they last until the permanent ban list is cleared or somebody removes you on the hub. It means that your Nickname / IP / CID is not allowed to connect to that hub any more, neither with DC++ nor any other DC client. Most hubs have reasonable owners and OP's that ban for logical reasons such as fake sharing or sharing illegal or unethical material (child pornography or nazi propaganda spring to mind). There are also those that will ban you for less reasonable things, such as your choice of client or because the OP has a sore tooth. If you run into the second type, the best thing you can do is to avoid it in the future. Who cares about them anyway? If you've been banned for reasons that fall into the first category, you've only got yourself to blame...