Backdoor programs are applications that open computers to access by remote systems. These programs typically respond to specially-built client programs but can be designed to respond to legitimate messaging applications.
A blacklist is a list of items in a set that are not accepted. This list is exclusionary, confirming that the item being analyzed is not acceptable. It is the opposite of a whitelist which confirms that items are acceptable.
A botnet (short for bot network) is a network of hijacked zombie computers controlled remotely by a hacker. The hacker uses the network to send spam and launch Denial of Service attacks, and may rent the network out to other cybercriminals.