Game ROMs must be placed within the
roms folder. It is recommanded to keep ROMs zipped, except for CD images.
Many users sort their ROMs into subfolders named after the system which the ROMs belong to. This results in hierarchy such as:
roms/ Atari - 2600/ Game 1.zip Game 2.zip Game 3.zip Sega - 32X/ Game 1.zip Game 2.zip etc. etc.
You are free to organize your ROMs as you prefer.
Lakka supports loading ROMs from an external USB drive.
It also supports loading ROMs from remote storage using network filesystems like NFS.
Lakka uses a ROM scanning system to generate the playlists in its horizontal menu. It is a convenient way to sort your ROMs on a per-system basis.
Each ROM stored in a folder being scanned by RetroArch is checked against a database.
This database is based on No-Intro and will recognize only the good dumps. See it as a sort of ROM validator.
If you would like to generate playlists manually, or generate playlists for systems that do not have scanning support yet, the playlist file format has been documented.
Cartdige ROMs from the famous consoles are well known and all follow the same rules:
But some systems are more complex.
The preferred format for CD images is BIN+CUE. The .cue is a text file like this:
FILE "NameOfTheBin.bin" BINARY TRACK 01 MODE2/2352 INDEX 01 00:00:00
For PSP games, the preferred format is ISO.
The specificity of Arcade games is explained here