Two taxes can apply: income tax (IR) and IOF. Whether you pay income tax depends on the security; IOF only applies if you withdraw before 30 days from purchase.
Income tax
CDB, LC, LF, Tesouro Direto and regular debentures are subject to income tax.
The tax applies only to your earnings – never to the amount you invested. It follows the regressive table, so the longer you stay invested, the lower the rate:
Up to 180 days: 22.5%
181 to 360 days: 20%
361 to 720 days: 17.5%
Over 720 days: 15%
The tax is withheld at source — at withdrawal, at maturity, or at coupon payment. You receive the amount already net and don't need to issue any payment form.
LCI, LCA, CRI and CRA are exempt from income tax for individuals.
IOF
IOF only applies if you withdraw less than 30 days after investing. It's charged on your earnings, on a regressive scale: 96% on day 1, falling to 0% on day 30. From day 30 onward there is no IOF at all.