Exascale Computing is the challenge of programming for the very large parallel computers which are likely to come about in the next few years. The name comes from the extrapolation of the floating point power in successive powers of ten, as represented by the top 500 computer installations in the world.
1 flop = 1 floating point calculation per second.
There has been a recent conference on this in Edinburgh, Scotland about this - see http://www.easc2013.org.uk/
See ProgrammingForParallelComputing