A K-1 is a tax document used in the United States to report income, deductions, and credits from partnerships, S corporations, estates, and trusts. It provides detailed information about each partner's or shareholder's share of the entity's income, which they must report on their personal tax returns. There are different types of K-1 forms, such as Form 1065 K-1 for partnerships and Form 1120S K-1 for S corporations. Each form includes specific information about the entity and the individual's share of profits or losses, helping ensure accurate tax reporting.