You can add individual files (either single files or multiple files in the usual Windows manner) as well as entire folders (directories) and the files will in principle stay there until you remove them. You can also save playlist-files. Open playlist (e.g. using [Alt]+[E] shortcut-key-combination) and then rightclick on the playlist to see the options.
P.S.:
Perhaps also check out playlist's preferences (default) options under
(rightclick >) "Options" > "Preferences"> "Playlist"