Trap doors: This is a way of bypassing normal authentication procedure (windows/ operating system user name  and password) to access a system. Once a system is compromised (impacted by) by malware, one or more backdoors may be installed for easier future access to the system.   

Logic Bombs/ Time Bombs:

Logic Bombs are not programs in their own right but rather camouflaged segments of other programs. They are not considered viruses because they do not replicate. But their objective is to destroy data on the computer once certain conditions have been met. Logic bombs go undetected until launched, and the results can be destructive. For example, some malicious programs are sot off during days such as April Fools Day or Friday the 13^th.