If you are checking your credit report yourself, by phone or online, it does not affect your credit report.
If a potential lender or creditor pulls your credit profile for the purpose of extending credit, however, it is called a "hard inquiry" and may affect your credit score. Multiple enquiries in a short period of time, like when you are shopping for a mortgage, are grouped together. The actual impact depends on the number of enquiries, time period and other factors. Each hard enquiry will be displayed on your credit profile for a minimum of six years.
If your credit report is generally good, a credit check will have less impact than if your credit report is generally bad. A consumer that is in some sort of financial difficulty may seek credit in an effort to "borrow" himself out of trouble. This will result in a large number of credit checks from different types of lenders on a consistent long-term basis. This activity will be reflected in his credit score as an indication of his financial difficulty.
So if you are concerned about the number of hard inquiries causing a negative impact on your credit report, you may want to apply for new credit in moderation.