FICO scores are a measure of credit risk calculated by Fair Isaac Corporation based on a credit report. Payment history makes up 35% of a score and factors like on-time payments, amount owed, length of credit history, new credit, and credit mix each influence the score. Individuals can improve their scores by paying bills on time, keeping credit utilization low, maintaining old accounts, and applying for new credit judiciously over time. A FICO score considers all these categories together to assess risk rather than any single factor.