An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions.
This allows him to:
Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
Prevent some or all transactions from gaining any confirmations
Prevent some or all other miners from mining any valid blocks
~18 hours ago this happened to Myriad. Cryptopool.eu which held more than 50% of the scrypt hashing power on the network forked.
What happened to the blockchain and the network ?
Nothing.
Because there were 4 other independent algorythms supporting the 49% on scrypt and proving that they were on the correct fork thus the fork had absolutely no effect on the network. Other than a few pissed off miners who lost a few hours of mining Myriad brushed off a 51% fork as if it were nothing.
This is why the concept of Myriad is the most inovative concept in crypto for a long time even above drk,x11 and hvc or you can name any other algo, because they cannot protect the network in any way all they do is keep away asics (for now), because hiding from ASICS is not the only problem the mining community has and fortunately Myriad solves most, if not all, of the current problems crypto coins are facing today.
I am not here to boast the coin and tell you to buy it, by all means don't buy it but look into it try to understand the concept and you will see as many senior/full members of this forum have seen that its concept is brilliant and it's the next step in the future of cryptocurrencies.
Relevant links are in my signature I am here to help you/answer any questions you may have to the extent of my knowledge (I am not the coin's developer, the coin's developer has better things like developing to do , he does waste his time hyping his coin which is another big + )
So wrong.