The ext2 or second extended filesystem is a file system for the Linux kernel. It was initially designed by Rémy Card as a replacement for the extended file system (ext).
The canonical implementation of ext2 is the ext2fs filesystem driver in the Linux kernel. Other implementations (of varying quality and completeness) exist in GNU Hurd, MINIX 3, Mac OS X (third-party), Darwin (same third-party as Mac OS X but untested), some BSD kernels, in Atari MiNT, and as third-party Microsoft Windows drivers.
ext2 was the default filesystem in several Linux distributions, including Debian and Red Hat Linux, until supplanted more recently by ext3, which is almost completely compatible with ext2 and is a journaling file system. ext2 is still the filesystem of choice for flash-based storage media (such as SD cards, and USB flash drives), since its lack of a journal minimizes the number of writes, and flash devices have a limited number of write cycles. Recent kernels, however, support a journal-less mode of ext4, which would offer the same benefit, along with a number of ext4-specific benefits.
Read more about Ext2: History, Ext2 Data Structures, File System Limits, Compression Extension