对不起 (duì bu qǐ) is the standard way to apologize in Chinese, used for both serious and minor apologies. It is stronger and more formal than 不好意思 (bù hǎo yì si), which is used for minor faults or to politely get attention. 对不起 can also mean 'to let (someone) down' (e.g., 我对不起你), a meaning that 不好意思 does not have.