The default file system of many Linux distributions is ext4. Its name means “fourth extended file system,” and it was designed as the successor to ext3, adding larger file-system capacities and performance improvements while retaining familiar Linux compatibility.
Ext4 is a journaling file system. Journaling records intended metadata changes before they are fully applied, helping the system recover more safely after a crash or sudden power loss. It also uses extents, which manage groups of adjacent disk blocks more efficiently than older block-mapping methods.
Ubuntu and Debian have historically used ext4 as a common installation default, which is why it is often treated as the standard Linux desktop file system. However, “default” is not universal: distributions and installers may offer or choose Btrfs, XFS, ZFS, or other systems depending on their goals. Btrfs, for example, adds snapshot and volume-management features, while FAT32 and NTFS are more strongly associated with removable media or Windows.