For starters, it totally changes the database performance paradigm for managing tables with many indexes.

The main technical underpinning (though note this isn't my area) seems to be basing indices on a particular improvement to B-trees, Cache-Oblivious Streaming B-Trees, which have some nice performance characteristics. I think what TokuDB calls "fractal trees" are just a catchier synonym.

Here's an academic paper from a few years ago from some of the people involved: http://supertech.csail.mit.edu/papers/sbtree.pdf

And a more recent talk focused on TokuDB: http://www.bnl.gov/csc/seminars/abstracts/Bender_Presentatio...