US salary figures are typically gross, and do not include bonuses, though a bonus may be mentioned in addition. Bonuses vary widely by industry and by the type of employer. Many jobs will have no bonus; others will have a small end-of-year bonus of similar value to a small gift ($50, say). Still others will have bonuses that will run into the thousands or hundreds of thousands of dollars.
Bonuses are typically variable, usually based on some measure or measures of performance, which is why they can't figure too prominently in a salary negotiation.
Income tax and payroll taxes are deducted from the gross figure. The employer pays additional payroll tax on top of the salary figure; this is not included in the figure you are quoted. For example, if your salary is $100,000, you and your employer will each have to pay $7,650 in social security and medicare taxes. After this deduction, your pay pay will be $92,350, but there are additional deductions to consider, including income tax withholding.
Income tax withholding will depend on the level of your salary, as well as your filing status and the number of dependents you have. These factors have a significant impact on your income tax liability, so they have a significant impact on tax withholding.
Some benefits may be deducted from your pay, as well. The amount of money employees need to contribute to their health care coverage varies from employer to employer. Some still have 100% employer-paid plans, but this is no longer common. Employee contributions are generally deducted before income tax is calculated. Employer contributions are not included in the gross salary figure, and the level of these contributions does not affect the employee's tax liability.
Defined-benefit pension schemes are rare these days. Instead, most employers have 401(k) plans, where the employee contributes from the paycheck. Most employers match up to a certain percentage (4% or 5%, for example) of the contribution. The tax arrangements vary; the classic 401(k) allows you to invest pre-tax income, and any earnings on those investments are taxed when they are withdrawn in retirement.
So, yes, if your gross annual salary is $120K, your gross monthly pay will be $10,000 a month, or $4,615 every two weeks (two-week pay periods are common). But your take-home pay will be considerably less than that.