There are a few different types of entries that cause you to a have a bad rating. They would include late payments that are recorded on your file such as credit card, personal loan, car and mortgage payments. When any of these types of accounts is more than thirty days late your score takes a hit.
Other types of accounts would include charge offs, collections, bankruptcies and foreclosures. Each of these results in a poor rating but the good news is that they are not to remain on your file forever. Each type of entry has a length of time after which it needs to be removed from your report. Unfortunately this does not happen automatically which is why you need to keep an eye on negative items on your report yourself.
For example, collection accounts are to be deleted seven years from the time of the first delinquency with the original creditor. So if you had a credit card that you missed a payment, and never made another payment, the seven year timeframe would begin from the date of the last payment you made.
Even if it was sent over to a third party collection agency, the account should be removed from your Equifax, Transunion and Experian files after the timeframe has expired. These items do not come off automatically and you would have to send a dispute letter to have them removed yourself.
Keep in mind that you do not have to wait that item frame to have every negative entry deleted. The bureaus are required to investigate any dispute that you send to them and if they are not verified with the creditor it needs to be erased from your file!