As of 2010, the IRS was conducting three kinds of audits. From least serious to most serious, they were the correspondence audit, the office audit and the field audit.
The IRS conducts a correspondence audit entirely by mail, usually because it just has a couple questions about your return. For an office audit, you'll have to bring all your paperwork to an IRS office for a meeting with the auditor. In a field audit, the auditor comes to you to look over your records--and possibly poke around your home or office looking for red flags.
If you're picked for a correspondence audit or office audit, you'll get a letter notifying you and telling you what to do next. If you've been picked for a field audit, you'll get a phone call.