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One thing people have to understand when dealing with UBIFS is that UBIFS is very different to any traditional file system - it does not work on top of block devices (like hard drives, MMC/SD cards, USB flash drives, SSDs, etc). UBIFS was designed to work on top of raw flash, which has nothing to do with block devices. This is why UBIFS does not work on MMC cards and the like - they look like block devices to the outside world because they implement FTL (Flash Translation Layer) support in hardware, which simply speaking emulates a block device on top of the built-in raw flash. Please, make sure you understand the differences between raw flash and, say, MMC flash before dealing with UBIFS. This section should help.

Overview
UBIFS is a new flash file system developed by Nokia engineers with help of the University of Szeged. In a way, UBIFS may be considered as the next generation of the JFFS2 file-system.
JFFS2 file system works on top of MTD devices, but UBIFS works on top of UBI volumes and cannot operate on top of MTD devices. In other words, there are 3 subsystems involved:
•   MTD subsystem, which provides uniform interface to access flash chips. MTD provides an notion of MTD devices (e.g., /dev/mtd0) which basically represents raw flash;
•   UBI subsystem, which is a wear-leveling and volume management system for flash devices; UBI works on top of MTD devices and provides a notion of UBI volumes; UBI volumes are higher level entities than MTD devices and they are devoid of many unpleasant issues MTD devices have (e.g., wearing and bad blocks); see here for more information;
•   UBIFS file system, which works on top of UBI volumes.

Here is a list of some of UBIFS features:
•   scalability - UBIFS scales well with respect to flash size; namely, mount time, memory consumption and I/O speed does not depend on flash size (currently it is not 100% true for memory consumption, but the dependency is very weak, and this may be fixed); UBIFS (not UBI!) should work fine for hundreds of GiB flashes; however, UBIFS depends on UBI which has scalability limitations (see here); nonetheless, the UBI/UBIFS stack scales much better than JFFS2, and if UBI becomes a bottleneck, it is always possible to implement UBI2 without changing UBIFS;
•   fast mount - unlike JFFS2, UBIFS does not have to scan whole media when mounting, it takes milliseconds for UBIFS to mount the media, and this does not depend on flash size; however, UBI initialization time depends on flash size and has to be taken into account (see here for more details);
•   write-back support - this dramatically improves the throughput of the file system in many workloads, comparing to JFFS2, which is write-through; see here for more details;
•   tolerance to unclean reboots - UBIFS is a journaling file system and it tolerates sudden crashes and unclean reboots; UBIFS just replays the journal and recovers from the unclean reboot; mount time is a little bit slower in this case, because of the need to replay the journal, but UBIFS does not need to scan whole media, so it anyway takes fractions of a second to mount UBIFS; note, authors payed special attention to this UBIFS aspect, see here;
•   fast I/O - even with write-back disabled (e.g., if UBIFS is mounted with the "-o sync" mount option) UBIFS shows good performance which is close to JFFS2 performance; bear in mind, it is extremely difficult to compete with JFFS2 in synchronous I/O, because JFFS2 does not maintain indexing data structures on flash, so it does not have the maintenance overhead, while UBIFS does have it; but UBIFS is still fast because of the way UBIFS commits the journal - it does not move the data physically from one place to another but instead, it just adds corresponding information to the file system index and picks different eraseblocks for the new journal (i.e., UBIFS has sort of "wandering" journal which constantly changes the position); there are other tricks like multi-headed journal which make UBIFS perform well;
•   on-the-flight compression - the data are stored in compressed form on the flash media, which makes it possible to put considerably more data to the flash than if the data were not compressed; this is very similar to what JFFS2 has; UBIFS also allows to switch the compression on/off on per-inode basis, which is very flexible; for example, one may switch the compression off by default and enable it only for certain files which are supposed to compress well; or one may switch compression on by default but disable it for supposedly uncompressible data like multimedia files; at the moment UBIFS supports only zlib and LZO compressors and it is not difficult to add more; see this section for more information.
•   recoverability - UBIFS may be fully recovered if the indexing information gets corrupted; each piece of information in UBIFS has a header which describes this piece of information, and it is possible to fully reconstruct the file system index by scanning the flash media; this is very similar to JFFS2; to make it more clear, imaging you have wiped out the FAT table on your FAT file system; for FAT FS this would be fatal; but if you similarly wipe out UBIFS index, you still may re-construct it, although a special user-space tool would be required to do this (this utility is not implemented at the moment, though);
•   integrity - UBIFS (as well as UBI) checksums everything it writes to the flash media to guarantee data integrity, UBIFS does not leave data or meta-data corruptions unnoticed (JFFS2 is doing the same); By default, UBIFS checks only meta-data CRC when it reads from the media, but not the data CRC; however, you may force CRC checking for data using one of the UBIFS mount options - see here.