Hack tools are programs used to alter the game rules in a game. In MMORPGs these programs usually alter the commands that the client sends to the server through the network and that's why they are called Network Hack Tools. Network Hack Tools also might mean programs used to illegally acces someone's computer throuth a network. People who do that are commonly known as hackers and... well, some of them just do it for fun but most are not that noble :)