Concepts

Command

Command is the beginning symbol used to determine the behavior of a set of command-line arguments.
The commands of a CLI program are usually in the form of a tree structure, and the program itself is the root command. Take git as an example, git is the root command and merge/commit/... are the sub-commands.
However, most commands doesn't have sub-commands because the root command is sufficient in most scenarios.
Command line parameters must start with the root command and list the subcommands in sequence along the path to a certain leaf command. When the target sub-command is listed, all subsequent symbols are regarded as the arguments or options of this sub-command.

Argument is the positional argument, which belongs to a command or an option.
The positional arguments are parsed according to the order in which they are specified. Take mv as an example, mv <src> <dest> shows that it has two positional arguments and the first will be recognized as source and the second as destination.

Option, usually a word or letter beginning with the - or / sign. (- is the Unix style and / is the Dos style)
Options can have none or a fixed number of positional arguments which are recommended to be non-required.