There are sort of a few ways to define "virtual terminal", a strict and more formal way and a more casual and perhaps common one (depending where you are...), the latter of which is really a colloquial synonym for terminal emulator or pseudo terminal.
The strict definition refers to the text consoles usually accessible (at least on linux systems) via AltCtrlF[1-6] -- actually F7+ may also work, but generally only the first 6 have a login prompt running on them, so anything beyond that will be a blank black screen (however, programs can still use them for output). If you look in
/dev these are
tty[N] devices (beware
tty___ is used as a prefix for other terminalesque things) and there are probably a lot more than just 6 or 7, or the number of F-keys.
These are called "virtual" because historically a single terminal was a physical piece of hardware combining a screen and keyboard. Virtual terminals probably require the OS to have access to such hardware in order to be useful, but they're not the hardware itself. They're software implementations of hardware protocols derived from very old school equipment (with some new school tweaks). Hence you can have an arbitrary number of them running with just one screen and keyboard.
Pseudo Terminals and Terminal Emulators
A pseudo terminal is a more general case of a virtual terminal. They're used for, e.g., remote logins and terminal emulators. Terminal emulator often refers specifically to the GUI window based programs you are familiar with (e.g., on Raspbian,
LXTerminal) but might also be construed to include that and virtual terminals proper.
openvt is called openvt because it is for opening a command via virtual terminals (VTs) in a strict sense, i.e., not including GUI emulators, remote login terminals, etc. These have some characteristics which place them closer to the actual hardware than most pseudo terminals. For example, the GUI desktop is actually run via a VT, and when a desktop is in use, it will occupy one. If you cycle through alt-ctrl-f[1-7] you should notice this; I used 7 instead of 6 because it is common to use the 7th one for this purpose (since it's usually the first one with no login prompt running), which is what it looks like that command targets:
sudo openvt -c 7 -s -f clear
-s switch means "Switch to the new VT when starting the command. The VT of the new command will be made the new current VT" (from
man openvt). So this might be used before running some other application, e.g., so any output it produces can't be confused with anything else, or, if it is a GUI application which can run on the framebuffer (i.e., without a formal desktop), so there is just a blank screen shown during the time it takes to load and start.