Unlike corporations, LLCs are much less formal in structure and how it is managed. The business structure of the LLC is very much the same as that of a partnership (or a sole proprietorship in the case of a single member LLC), but with the limited liability protection, discussed above. The management of the LLC is typically invested in all of the LLC members, which is the generic term for the LLC owners.
The LLC members or owners are the ultimate authority, and are responsible for all aspects of the LLC. There are no separate positions like there are with a corporation (which requires shareholders, directors, and officers). An LLC member is like all three of those corporate roles in one (although LLC members may certainly have any titles they deem appropriate, and may agree to divide up various responsibilities in whatever way the members see fit); all members have complete authority over the LLC.
The LLC ownership is typically expressed in percentages (e.g. Sara and Jim each own 50% of S & J ENTERPRISES LLC) as opposed to the shares of stock that a corporation would have. Another business entity can even be a member of an LLC.