Company culture, in basic terms, refers to what it's like to work for a particular employer. Though cultures vary from place to place, it is important that you can quickly assess and mesh with the organization which you will be spending much of your waking life with.
Whether the norm is a 3 piece suit or flip flops, make sure that your wardrobe is in line with expectations. You will probably be dressing differently from how you are accustomed to at school. Dress codes exist for a reason, and it's important to abide by them.
Adherence to Time
Some jobs are strictly from 8-5, while others have varying start and end times. Find out what is expected of you, and don't assume anything. People's work pace and time management skills vary - does your co-worker stay until 7pm because it's expected, or does she not work efficiently and waste too much time socializing? Also note the norms as they relate to when meetings start and end - do you come right in and get down to business, or is small talk and socializing expected before a meeting begins?
Does your supervisor prefer that you follow up via email, IM, phone? Do you just pop-in when you have a question, or do you schedule a meeting? Are emails expected to be read and responded to ASAP, or by COB (close of business)? These are things that you need to know. So if you are unsure..ask!
How does your new employer values and recognizes diversity? At UNC, we typically enjoy an open, supportive, and inclusive environment; there are people here from all races, religions, sexual orientations, nationalities, and political affiliations. This may not be the case at your new workplace. While some people may be particularly outspoken about their beliefs and opinions, the workplace is not usually a good setting for controversial conversations, especially as a new hire. With that said, if you experience something that makes you feel uncomfortable, it is important to know what your options are. Talk with your supervisor or HR department - many organizations have helpful resources available. No one should be made to feel uncomfortable or slighted at work due to their beliefs or affinities.
Misc Office Norms
What is the norm on forwarding email jokes, or asking for donations for a fundraiser, or starting an NCAA bracket, or getting together for drinks after work? Some of these may be clearly outlined in a company training manual, others may just be learned OTJ. Again, if you are unsure, then ask.