1. The simplest is the correspondence audit. You will get a letter from the IRS requesting that additional information be mailed to them or that a proposed additional amount of tax be sent in.
2. The second level of audit is the office audit. You will be asked to bring certain information to the IRS office for review.
3. The most complete audits are called field audits. They are conducted at your place of business. It is best to avoid these if possible. Once the agent is at your place, he or she has much more to see and ask about.
If you receive a notice from the IRS, do not ignore it. You can rest assured that the IRS will not just go away. All contacts by the IRS should be handled promptly. Unless you are an accomplished IRS "fighter", you would be wise to seek professional assistance from the very first correspondence.
Getting "chatty" with an IRS agent or providing information or documents which you were not asked to provide is ill-advised. Such excess information may expand the audit into areas which were not of initial interest to the agent. Your attempts at being friendly with the agent may end up costing you more time and money than is necessary.
On the other hand, the only taxpayers who need to fear the IRS agent are those who have cheated on their tax return or who can't provide the documents necessary to support the numbers on their return.