You can be fired for any legal reason
In at-will employment, your employer can fire you for any number of reasons -- basically anything that isn't specifically illegal. For example, you can be fired for coming into work late, but you cannot be fired because you are pregnant. Note that just because you are pregnant doesn't mean you can't be fired -- it just can't be the reason for your termination.
Illegal reasons for firing an employee
You cannot be fired based on the following: race, sex, religion, disability, citizenship, pregnancy, etc. It is illegal for an employer to base your termination on these factors. Also, an employee cannot be fired for filing a workers' compensation claim. Firing an employee for these reasons can lead to a wrongful termination claim.
It goes both ways
For the employee, the at-will employment law gives you flexibility when leaving a job. While you may choose to do so, you are not required to give your employer notice before you quit. Many employees give two weeks notice, but it's not required by law.
Illinois law assumes that every employee is an at-will employee, but there are exceptions. If you have an employment contract, you may be guaranteed employment for a certain amount of time or perhaps there are only specific reasons why you can be fired; it depends on the contract. Other situations where an employee may not be at will is in union contracts or where some other form of agreement has been made between employer and employee.