Formal Discovery is a legal process that begins once a lawsuit is filed.  In some cases discovery may begin before discovery but I won't address that here.  Discovery encompasses a wide variety of investigative tools including the following:

• Interrogatories--Governed by Illinois Supreme Court Rule 213, written questions, usually limited to 33, directed at the opposing party.  The answers must be made in writing, under oath and within 28 days.  These answers may be utilized during trial to impeach testimony, so it is very important to answer these questions truthfully and with the help of an experienced attorney. 
  
• Request For Production of Documents-Governed by Illinois Supreme Court Rule 214, a request from the opposing party for specific documents which are relevant to the case or may lead to relevant information.  These documents must be produced within 28 days and a signed sworn statement must be attached testifying that the production of documents are complete and accurate.  If documents aren't produced, you may be prohibited from using the documents at trial.
• Request For Admissions--Governed by Illinois Supreme Court Rule 216, written statements propounded to the opposing party in which they must either agree, disagree or state they do not have enough information to answer the question.  These must be answered within 28 days. 
  
• Depositions-Governed by Illinois Supreme Court Rules 201, 202, 203, 204, 205, 206, 207 and 212.  Oral questions which must be answered under oath.  A deposition may last up to three hours and may be video taped.