cp -R ./folderA/* ./folderB/
If two files and one folder are inside folderA, they will be copied to folderB after issuing the command.
-R means recursively copying the files and folders. * means everything inside the folder. "." means the current directory.
Technical things: Directories are referred to as folders. Folders are referred to as directories. They are used interchangeably.
Linux and UNIX cp command help