In many clients and online wallets, there exist a feature called ``accounts'', which splits your available balance into various buckets.
Addresses have corresponding accounts: money sent to these addresses will increment the balance of the corresponding account. On the other hand, accounts do not have associated addresses. Specifically, spending money from an account decrements the balance of that account, but does not restrict which available leaf nodes are used. (Remember, there is no such thing as a sending address. It makes no sense for send-from-account transactions to only spend ``from'' the associated address.)
Similarly, moving value between accounts is purely an accounting activity. It does not touch the network. It has nothing to do with addresses.
Because they are merely an accounting tool, it is possible for accounts to have negative balances. The network itself never works with negative numbers.