A 51% attack is a situation where more than half of the network's hashpower is in the hands of a single individual or concentrated group. The "attacker(s)" would be able to prevent new transactions from gaining confirmations, allowing them to halt payments between some or all users. They would also be able to reverse transactions that were completed while they were in control of the network, meaning they could double-spend coins.
The "attacker(s)" although won't be able to create new coins or alter old blocks, so a 51% attack would probably not destroy a particular cryptocurrency or another blockchain-based currency outright, even if it proved highly damaging.
