Read the wiki FAQ by Alexgr it is gold.http://wiki.darkcoin.eu/wiki/Main_PageWhat are Master Nodes?
Masternodes are network nodes, owned by the users of Darkcoin network. They perform the coin mixing service of DarkSend which anonymizes transactions. Users get rewarded with 10% of the mining output per block (without actually performing any mining themselves) through a "voting mechanism".
Masternodes require 1000 DRK to be set up as a negative incentive that disallows one party running or controlling too many of them.
In the future Masternodes, or the Masternode-payment system, could possibly be utilized for providing other kind of services, like blockchain hosting for thin-client functionality.
The masternode payment protocol will be implemented on the main Darkcoin network on May 25th, as Release Candidate status. For the time being over 100 masternodes operate on the Darkcoin network, however they are not receiving any payments.