These are often held, pre-interview day, by employers who have scheduled on-campus interviews, to help students be prepared.
Employer information sessions are typically held the evening prior to an employer's first interview day through On-Campus Interviewing. However, some employers hold them in conjunction with attending career fairs, or with other reasons they have personnel on campus.
Learn more about the organization and meet company representatives in a less formal situation than an interview. You have the opportunity to ask questions in advance of the interview.
Consider the employer's perspective on interviews:
They sit in a room all day, and conducts 13 30-minute interviews with 13 students. The employer would rather use that time to get to know you, not repeat info about their company 13 times. Information sessions let employers cover that information once and give students the chance to think over the information before interviews. Students meet someone from the organization (might be your interviewer or someone else) and it breaks the ice. If you don't go before an interview and you don't have a good reason why, it tells the employer you really aren't that interested.
Most are 90 minutes to two hours. Sometimes they are formal presentations. Sometimes they are receptions where you can arrive and depart throughout the session. If you need to arrive late or can't stay for the full duration, contact the employer representative in advance and let them know. Employers won't mind if you show the courtesy and interest by getting in touch. They'll be glad you're interested in attending.
Often any student interested in the employer is welcome to attend advertised information sessions, regardless of whether you have an interview scheduled. Some sessions, particularly receptions, are by invitation only. If you have heard about a session but can't find the public announcement, contact the employer and ask if it's okay to attend.
You can still go; you might be able to arrange an interview. Even if you tried, but didn't get an interview, if you are very interested in the employer, go to the information session, take your resume, introduce yourself, and explain that you would very much like to interview if there is any possibility of doing so. The employer might have had a cancellation on the interview schedule and be willing to work you in or make other arrangements to meet with you. Maybe your resume didn't stand out from the crowd, but you can present yourself well in person. Employers are impressed by initiative, so this often is effective. The worst possibility is that the employer says no; you haven't lost anything.
Business casual unless the invitation or announcement says otherwise. Events on-campus are usually less formal than events in hotels. Some employers may specify business / interview attire for receptions in hotels. If your schedule doesn't permit you to change into appropriate attire, contact the employer in advance and ask if that's okay. Basically the employer wants you to attend, and wants it to be convenient for you to attend; they also wants to observe your judgement in choosing your attire.