Account Sharing is when multiple people share a single RuneScape account. This directly violates the 'Buying or selling an account' rule under Honour, and the punishment for all parties involved in sharing accounts is generally a permanent ban. Account sharing is unfair because it allows an account to train and be used more often than a normal human's schedule and free time would allow, enabling the account to move up quickly in skill levels and hiscore ranks and accumulate large amounts of money easily. Shared accounts are sometimes mistaken to be macros.
Shared accounts can be recognized a number of ways. Often, players who have a shared account on their friends list will notice that the player using the shared account seems to forget previous conversations, possibly contradict themself, and ask about things that they seemed to know before. JaGeX also can recognize shared accounts because the account will quickly switch between two different computers or IP addresses.