I am trying to understand what is considered income while receiving social security benefits? I know W2 income is, but are there other forms of income such as 1099, rental income, etc?

Your SS benefits may be affected by the amount of your "earned income," which basically means salary, wages and tips. It is not affected by unearned income - such as interest, dividends, capital gains, pensions, or withdrawals from your retirement savings plans - IRA, 401(k), or 403(b) for example. So it depends on what type of 1099 you're talking about - if you are a contractor and receive a 1099 from a client, then it's earned income. But if it's a 1099-INT, 1099-DIV, 1099-R, or 1099-B - then it's not.