Go to Financial Information System -> General Ledger -> Chart of Accounts Maintenance (GL0018)
On the General Ledger Chart of Accounts Maintenance screen it will give you an option once you have filled out the top section on what you want the status of the account to be.
When you make an account invalid it will not allow you to apply transactions. The account will no longer show up on some reports and cannot be posted to. This is for an account that is no longer in use.
When you make an account inactive you can still enter the budget amounts but you can not post transactions to it.