It's not you, it's the great American tax system that is confusing. In your case there are several rules that add to the confusion. But, as answered by Rick 19744; all income is reportable and taxable, unless specifically exempt by law (e.g. VA benefits). There is no dollar amount threshold.
Most of the confusion, in your situation, comes from these two rules:
1. Businesses are required to issue a 1099-MISC if they pay a contract employee/subcontractor more than $600. This is frequently interpreted as meaning you don't have to file the income. That's not true. What it really means is you can probably get away with not reporting it because the IRS doesn't know about it, thru the regular tax document system (1099's).
2. You are not subject to self employment tax (SET) if you made less than $400. SET is how the the self employed (and that is what you are considered to be) pay their social security & medicare tax. The $100 you make is subject to income tax, but not SET. Since your amount is less than $400, you may report the income as misc. income, instead of Self employment, simplifying the reporting of the income. The "long form", form 1040 will still be required.
