A backdoor is an unauthorised tool that has been installed on a syste after a compromise, thereby allowing  an attacker gain easily access to the system in the future without being detected by security mechanisms that are in place.

Generally refers to the allocated capacity of a communication channel to pass data within a given amount of time. Bandwidth is normally expressed as bits per second (bps), Kbps, Mpbs...etc.

Botnet means "Bot Network" and represents a group of hijacked computers that are controlled remotely by a hacker. Botnets are used to send spam emails and/or launch Denial of Service attacks by cybercriminals.