You have to pay income taxes on cap gains. For securities held for less than a year (short term cap gain/loss) is taxed at ordinary income rates. For securites healf for more than a year (long term cap gains/loss) they are taxed at a more favorable long term cap gains rate (15%).
As far as reporting, your broker will generate a 1099 tax form and send it to you. That form is also sent to the IRS. The IRS can then comaire your return with the info they got from your broker to determine if you claimed you cap gains.
edit: Interest income (like from your bank account) is also taxable income.