A virtual machine is the emulation of an operating system using the actual hardware resources of the machine where the emulation is running.
CentOS is focused on servers, so it is more optimized for it, but it is more complicated to use.
Debian is focused on a user environment, so it is less optimized for server use but easier to use.
Advanced Package Tool (apt) is the package manager used in Debian, it is in charge of managing all packages and package repositories needed by the system, either to install new programs/dependencies or to update existing ones on the system.
Aptitude is an improved successor to apt that contains new commands and a graphical interface from which to view and manage packages/dependencies.
Apparmor is a Linux kernel security module that allows the system administrator to restrict the capabilities of a program, i.e. a permissions manager for programs.
The use of password policies helps computer security by keeping the computer less vulnerable to simple or common passwords.
Logical Volume Manager (LVM) is an implementation of a logical volume manager for the Linux kernel. Its main feature is that you can modify volumes without rebooting the system.
Superuser do (sudo) is a Unix-like operating system utility that allows users to run programs with the security privileges of another user in a secure manner, thus temporarily becoming the other user during program execution.
Uncomplicated Firewall (UFW) is an application that aims to set rules in "iptables", the native firewall tables in Linux. We use it to manage in a simple way the firewall rules of the system, protecting it from unauthorized access from the network.
Secure Shell (SSH) is a protocol whose main function is to remotely access a server through a secure channel in which all information is encrypted. It is used to remotely and securely connect to systems.