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I know both are a type of malware, trojans 'disguise them self as a useful program' and rootkits 'embed them self in the system to avoid detection'. Technically how do they accomplish such things? I'm confused because all viruses avoid detection in the sense that they don't display some EULA saying "this is a virus that's going to screw up your computer, click agree to continue".
For example if an attacker emailed a file to a victim claiming it was just an invoice, but in fact it was a virus, would that be considered a trojan?
I know different companies use these words differently, so is it even a worth while endeavor to try to understand them in depth?