To record income you do not wish to send as an invoice you can create an invoice and keep this on the system. Either way, the system will record and maintain a record of income received and log this as taxable income.
Tip - To record income not assigned to a client, create a client called 'non-invoiced' or similar and choose this when selecting a client for the invoice.
To record income that may fall under a specific taxable category you can choose 'Other income' in the income drop down menu.