In the United States, credit reports are compiled by three credit bureaus — Equifax, Experian, and TransUnion. These three major credit unions use the FICO scoring system to determine credit scores. As credit history data compiled may vary from one credit bureau to the next, so too may credit scores.

The FICO score can range from 300―900 and the mean score is 723. Fine, but just what exactly does that mean? Your FICO score is determined by the following factors:

35% Payment history — Do you pay your bills on time? Any late bill payments, particularly those that are more than 30 days late could adversely affect this section of your score.
30% Amounts owed — How much outstanding debt have you accrued? How much do you owe on car loans or home loans? The more you owe, the lower this section of your score will be. Also included in this section is the amount of cards you have at their limits.
15% Length of credit history — Do you have a long established credit history? Generally, more data in your credit report helps your score by increasing the reliability of that data. Too short a credit history does not give credit agencies a very good look at your long-term habits.
10% New credit — Have you had many recent inquries on your credit report? An exorbitant amount of recent inquiries could mean that you're experiencing financial trouble. This could lower your credit score.
10% Types of credit used — How many loans and credit cards do you currently have? Too many credit cards or loans from finance companies, like the ones you see on TV at 2:00 a.m., could negatively affect your score.

The exact algorithm used to calculate credit score is confidential proprietary information owned by Fair Isaac. But carefully considering the points laid out above should help you take the necessary steps to improve your score.