They are all 16/32 bit hybrid kernels. They use MS-DOS as a boot loader and as a compatibility layer for legacy applications, meaning they can run DOS and Windows 3.x programs natively with little or no compatibility errors.
They are all part of the Windows 4.0 production line, meaning their internal version numbers for each version of Windows read like 4.0.950 (original 95 release), 4.10.1998 (original 98 release), and 4.90.3000 (original ME release).
Unlike Windows NT, Windows could not use NTFS as their file system of choice barring certain third party utilities, which meant that 95 (starting with 95B or OSR 2.1), 98, and ME could only use FAT32 (which allowed hard drives of up to 32 GB in size compared to a mere 2 GB for FAT), and since they used MS-DOS memory calls that lacked file permissions, this meant the Windows 9.x series was often prone to file system corruption and hard drive issues.
The entire line of 9x Windows had better support for USB drivers, gaming support, and multimedia, which made it a popular choice for consumers. Windows NT lacked most of these features or supported them poorly, but had better stability and security. Windows 2000 was the first version of NT to start bridging this gap, and with the release of Windows XP, Microsoft discontinued the 9x line altogether since Windows XP was a synthesis of the best aspects of the 9x and NT line of operating systems.