If you decided to open a new business that will be based in Illinois you can choose from several options:
Sole Proprietorship: Sole owners of Illinois-based businesses could opt for sole proprietorship as the easiest form of business organization. Not the most recommended, given the liability a sole proprietor assumes as a result of owning a business. No registration with Illinois Secretary of State is necessary, but it is recommended to register an Assumed Business Name (DBA), and if you plan to hire employees then also obtain an EIN.
REMEMBER: Illinois requires to publish the Assumed Business Name Certificate of Registration Notice in a general circulation newspaper in the county of registration. Learn more about Illinois publication requirements here.
Single Member LLC: Limited liability company, as the name suggests, is an entity that allows its owners to limit the liability of the business to the entity itself, shielding the owners' personal assets. This type of entity is recommended for most small businesses.
By default your LLC will be taxed as "disregarded entity", meaning you will file your LLC tax return as part of your personal tax return. Keep in mind though - LLC is a flexible entity, which means you have the option of electing it to be taxed as S-Corp (assuming you are a U.S. person) or C-Corp. Learn more about LLC here, and about the details of forming LLC in Illinois here.
KEEP IN MIND: Illinois is one of the two states (the other is New York) where the cost of forming LLC is much higher than cost of forming a corporaiton. The reason for such price difference in Illinois is simply the difference in state filing fees - $325 as of this writing.
Corporation: You can also form a corporation and be a sole shareholder with 100% of all shares. Corporations have more formalities than LLCs (for example in Illinois you are required to have bylaws and maintain minutes of meetings in corporate records), but provide similar limited liability protection. That's one of the reasons this entity type is often more suitable for bigger companies, or those who seek major investment.
Corporations can be taxed as S-Corp or C-Corp, with each form of taxation having its pros and cons. Keep in mind, you can elect your corporation to be S-Corp only if you, as the sole shareholder, are a U.S. person.
General Partnership: Like sole proprietorship, this entity type does not require registration with the Illinois Secretary of State, but it also does not protect the owners from business liability, and therefore is usually not recommended. A General Partnership needs to register an Assumed Business Name (DBA), and obtain an EIN.
Multiple Member LLC: like Single Member LLC for sole owner, Multiple Member LLC is often the entity of choice for small and new businesses with more than one partner.
Corporation: Since corporation can have many shareholders, and transferring ownership is relatively easy (though share transfer) corporation might be a good choice of entity for business with partners.
Keep in mind though - S Corporations are limited to 100 shareholders who must be physical U.S. persons. That means corporations owned (partially or fully) by non-U.S. persons or legal entities, cannot be elected as S-Corp, and therefore subject to double taxation of an C-Corp. In cases like that it would be recommended to consider choosing LLC instead.
Limited Partnerships: Limited partnerships come in different forms, depending on the state (LP, LLP, LLLP). Though Limited Partnerships have their own purpose and place, for most cases we believe an LLC would serve its owners well enough, therefore at this point we do not cover Limited Partnerships.
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