Marlin is the default file manager for Elementary OS, developed by ammonkey. It is now discontinued and will eventually be replaced by Pantheon-Files.
Marlin is the successor to Nautilus Elementary. It is a file manager with a GTK+3 interface "with a focus on speed, simplicity, ease of use."
ammonkey, Marlin's author, has lost interest in the project and as such Marlin is no longer being developed. Instead, development effort will be focused on Pantheon-Files, intended to be Marlin's replacement for elementary OS in the future.