The LLC form is different for different states, with some real advantages in some states that aren’t relevant in others.
An LLC is usually a lot like an S corporation, a combination of some limitation on legal liability and some favorable tax treatment for profits and transfer of assets. This is a newer form of legal entity, and often harder to establish than a corporation.
Why would you establish an LLC instead of a corporation? That’s a tough legal question, not one we can answer here. In general, the LLC has to be missing two of the four characteristics of a corporation (limited liability, centralized management, continuity of life, and free transferability of ownership interest). Still, with the advisability and advantages varying from state to state, here again, this is a question to take to a good local attorney with small business experience.
For more about LLCs and the business structures you have available to you, check out: How to Form an LLC, and The Complete Guide to Choosing Your Business Structure.