1. Hard inquiries.
These are when you apply for a new credit card or loan, and the agency looks closely at your history to see how much of a risk you would be if they gave you credit. These are inquiries initiated by you (by applying) and they are the most damaging to your score. Ten percent of your score is affected by inquiries. There is nothing you can do about these; since you approved it, it will stay on your report and negatively affect it for at least six months, but then begin to diminish. After two years it comes off your report altogether.

2. Soft inquiries.
These are when a creditor you are currently a customer of checks your credit history, perhaps to verify information, or when you check it yourself. These do not affect your score and thus you should not worry about them.

3. Promotional inquiries.
Like soft, these will not affect your score. These are from companies who want to send you "pre-approved" cards and such and need a cursory look at your history to see if you are in fact worthy of pre-approval.

4. Unauthorized inquiries.
I had my credit limit reduced once without warning, and when I asked about it, the company told me it was because of "too many inquiries." When I looked at my credit report, sure enough, there were several hard inquiries on there that weren't related to any applications I had filled out that I was aware of. These types of inquiries are absolutely disputable (after all, someone is hurting your score without your consent!), and you should write letters to the inquiring agencies requesting proof that you approved them to do this. The one exception to this is collection agencies. They can perform hard inquiries without your consent because you owe them debt. You can't dispute these.