An LLC is a business entity formed and regulated under the laws of the state where it was created. For tax purposes, a single-member LLC (SMLLC), by default, is treated as a disregarded entity. This means that you would file a Sch. C with your individual income tax return. A multi-member LLC (MMLLC), by default, is treated as a partnership. This means that you would file a partnership return, and any income, loss, credits, etc. would flow through to you and would be reflected on your individual income tax return via Sch. K-1 from the partnership.
Both the SMLLC & MMLLC can elect to be treated as a corporation and taxed as such. As a corporation, you can elect to be taxed as an S Corporation and any income, loss, credits, etc. would flow through to you and would be reflected on your individual income tax return via Sch. K-1 from the S Corp.
The sole proprietorship, unlike the LLC or S Corp., would have unlimited liability, meaning that your personal assets could be used to satisfy business debts and actions.
The sole proprietorship's net income would be subject to both personal income tax and self-employment tax.
A partner/member in a MMLLC, taxed as a partnership, would be tax taxed on their share of the business income (including guaranteed payments). That income and guaranteed payments would be subject to both personal income tax and self-employment tax.
An employee-shareholder in either a SMLLC or a MMLLC would receive wages for services rendered (subject to FICA taxes). The wages paid to the employee-shareholder must be reasonable and are subject to personal income tax; however, the additional profits passed through to the shareholder are taxable only for income tax purposes but not for self-employment tax.
The following link to the Small Business Administration discusses Business Structures in more detail: https://www.sba.gov/business-guide/launch-your-business/choose-business-structure
The following link to the IRS discusses Business Structures in more detail as well: https://www.irs.gov/businesses/small-businesses-self-employed/business-structures
