The short answer
Venue is which courthouse hears the case. In Illinois, a case generally belongs in the county where a defendant resides or where the transaction occurred. Cases can sometimes be transferred to a more appropriate county or federal district.
Where it fits in your case
An objection to improper venue must be raised early, generally by the time to answer.
What to expect
Improper venue can be cured by transfer to a proper county (735 ILCS 5/2-101 and 2-104).
Forum non conveniens allows transfer when another forum is substantially more convenient.
Federal transfers follow 28 U.S.C. § 1404(a), and a valid forum selection clause usually controls.
What we need from you
Contracts with forum clauses, where witnesses and documents are located, and where the key events happened.
Common misconceptions
“The plaintiff’s choice is final.” Transfer is possible, though a plaintiff’s home county gets deference.
“Venue decides who wins.” It mostly affects cost and convenience.