The term 'rootkit' originally comes from the Unix world, where the word 'root' is used to describe a user with the highest possible level of access privileges, similar to an 'Administrator' in Windows. The word 'kit' refers to the software that grants root-level access to the machine. Put the two together and you get 'rootkit', a program that gives someone - with legitimate or malicious intentions - privileged access to a computer.
There are four main types of rootkits: Kernel rootkits, User mode rootkits, Bootloader rootkits, Memory rootkits

ETHOS is the Cisco file grouping engine. It allows us to group families of files together so if we see variants of a malware, we mark the ETHOS hash as malicious and whole families of malware are instantly detected.
Reference:
ETHOS = Fuzzy Fingerprinting using static/passive heuristics

A buffer overflow (or buffer overrun) occurs when the volume of data exceeds the storage capacity of the memory buffer. As a result, the program attempting to write the data to the buffer overwrites adjacent memory locations.
Buffer overflow is a vulnerability in low level codes of C and C++. An attacker can cause the program to crash, make data corrupt, steal some private information or run his/her own code. It basically means to access any buffer outside of it's alloted memory space. This happens quite frequently in the case of arrays.