In the same way that the U.S. Constitution set up the general structure of the federal government, each state has needed to enact its a constitution to establish its own governmental structure.
The Illinois Constitution was first adopted in 1818, shortly before it became a state. It was modeled somewhat after the U.S. Constitution, which means that the organization of Illinois' government is similar to that of the federal government. Both have three branches of government with similar functions, and both have a bill of rights.
In Illinois, the executive branch is headed by the governor, the legislative branch is called the General Assembly, and the judicial branch consists of the Illinois Supreme Court and the courts below it.
Like the U.S. Constitution, the Illinois Constitution provides the means for amending it. The Illinois Constitution can be amended either by Constitutional Convention (if 3/5 of the members in each House of the General Assembly agree to it, which voters can approve or disapprove) or by the General Assembly (if 3/5 of each house of the General Assembly approve the amendment, which is then submitted to the voters at the next general election).
The 1818 Constitution reflected the fact that Illinois was a frontier state at the time. As Illinois has grown and changed, the Constitution has been amended to reflect more modern needs. New constitutions have been adopted in 1848, 1870, and 1970. Prior to the 1970 Constitution, for example, the General Assembly could impose only property, occupation, and franchise and privilege taxes, and it could not enter into a contract debt in excess of $250,000.
The Illinois Constitution is available at http://www.ilga.gov/commission/lrb/conmain.htm
This IS NOT intended to be legal advice or in any way replace the advice and judgment of a licensed lawyer. Every case and situation is unique and only a licensed lawyer can offer legal advice which is appropriate for your situation