While an LLC with more than one member will be treated as a partnership for tax purposes, it has the right to elect to be treated as a corporation, either a regular of "C" corporation, or an electing corporation under Chapter S of the Internal Revenue Code. If you have an LLC that desires to do an S-Corp election, just fill out the 2553 form (S Election) as normal – but at the top of the form write “Pursuant to Rev. Proc. 2004-48” – if you are mailing it in, then it needs to be in red ink, but if faxing it doesn’t matter. If you make the S election, then you do not need to fill out the 8832 (Check-a-Box) form. If you wish to elect corporate status, but not S corporation, then file form 8832.
A single member LLC cannot be treated as either a partnership, or a corporation. It must be treated as a sole proprietorship for tax purposes. The IRS calls single member LLCs "disregarded entities." The file no tax return, rather their operations are reported on Schedule C of the member's income tax return. I’m not sure that all states permit single member LLCs, but Illinois, where I practice, does.
An LLC, whether with a single member or multiple, can elect (subject to certain restrictions) to be taxed as a C corporation or an S corporation. Otherwise, it is treated as a sole proprietorship if it is has one member or a partnership if it has multiple members.
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The foregoing is general information only, not specific legal advice. No attorney/client relation has been created or should be implied.