Files don't stay where you put them forever. You'll need to copy them to new locations, move them between directories, or rename them. Two commands cover all of this: cp for copying and mv for moving.
Copying files — cp
cp takes two arguments: the source file and the destination.
$ cp notes.txt notes-backup.txt
$ ls
notes.txt notes-backup.txt
Now you have two identical files. If you copy to a directory, the file keeps its name:
$ cp notes.txt Documents/
$ ls Documents/
notes.txt
Copying directories — cp -r
Files are easy, but directories need the -r flag (recursive) to copy everything inside them.
$ cp -r projects/ projects-backup/
$ ls
projects projects-backup
Without -r, cp will refuse to copy a directory. The -r tells it to walk through every subdirectory and file inside.
Moving and renaming — mv
mv moves a file from one place to another. It also renames files — because renaming is really just moving a file to a new name in the same directory.
$ mv notes.txt old-notes.txt
$ ls
old-notes.txt
Moving to a different directory works the same way:
$ mv old-notes.txt Documents/
$ ls Documents/
old-notes.txt
You can also move and rename in one step:
$ mv Documents/old-notes.txt ./notes.txt
That moves the file back to the current directory (.) and renames it.
What's the difference?
cp leaves the original file where it is and makes a duplicate. mv picks the file up and drops it somewhere else — the original is gone. Think of cp as "copy-paste" and mv as "cut-paste."
One more thing: mv also works on directories without a special flag. mv projects/ old-projects/ renames the directory just like it would a file.