In the United States, LLC stands for "limited liability company." It has been mistakenly referred to by some as a "limited liability corporation", but that is a misnomer. An LLC is NOT a corporation; rather it is an unincorporated association. An LLC is organized--not incorporated--at the state level by filing articles of organization with the appropriate state agency.
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There is no such thing as an LLCompany or LLCorporation in the United States. An LLC is simply a limited liability company, i.e., a legal entity organized at the state level that generally provides liability protection for its owners against the debts and obligations of the company. The nature and degree of such limited liability protection are dependent on the statutes governing limited liability companies in the state of organization. Taxation of an LLC is a separate matter altogether.

An LLC is a pass-through entity for tax purposes, thus sayeth the IRS. Specifically, a single-member LLC is a "disregarded entity" for tax purposes and is taxed as a sole proprietorship by default. A single-member LLC that accepts the default tax classification is NOT an entity that is taxed separately by the IRS. The business profit or loss of a single-member LLC is reported on Schedule C which is attached the owner's 1040 individual income tax return. Hence, the pass-through taxation.

A multi-member LLC is taxed as a partnership by default. Multi-member LLCs therefore file 1065 partnership returns. Ultimately, the profit or loss of such a multi-member LLC passes through to its owners (partners) in proportion to their percentage of ownership interest in the LLC. Again, pass-through taxation.

An LLC that does not wish to accept the default tax classification may elect to be taxed as 1) a corporation, by filing IRS Form 8832, or 2) an S corporation, by filing IRS Form 2553. Whether or not an S corp election needs to be made at the state level depends entirely on the state. Not all states have an equivalent S corp election form; some states simply accept the LLC's S corp tax status based on the IRS's approval of the election made on Form 2553.

An S corp, although it also provides pass-through taxation, can present a separate set of tax implications that is different from that of an LLC (including but not limited to differences related to capital gains, employment taxes, home office expense deductibility, and health insurance deductibility for certain owners). This not to say that one pass-through entity is better than the other overall; every situation is different and requires thoughtful consideration when selecting a business entity.

An LLC is not an LLP. Even a multi-member LLC (or "partnership LLC", for lack of a better term) is not an LLP. LLP stands for limited liability partnership, a completely different type of business structure. The nature and degree of limited liability protection afforded by an LLP are dependent on the statutes governing LLPs in the state of registration. Some states bestow a degree of liability protection that is similar to that of an LLC, while others do not. In some states, for example, a partner in an LLP may be shielded from or indemnified against the liability resulting from the improper actions of another partner only (but not necessarily against the contractual liability of the LLP overall).

It would appear that the reader would benefit greatly from a consultation with a qualified CPA or attorney (or both) for assistance in gaining an understanding of the characteristics of the various business structures and associated tax implications. Such a consultation is particularly encouraged--strongly encouraged--if the reader is attempting to advise clients of his own on these matters. In addition to the various IRS publications, recommended reading would include Nolo.com's discussion of the various business entities at
http://www.nolo.com/article.cfm/objectID/2C1A52D5-9364-4F92-AD191B56DF9D4CE6

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