Would it be okay to contact a professor from the school you wish to attend....

[Just Jeff]

and ask them about the department?

[fuzzylogician]

Yes, as long as the questions you want to ask have not already been answered on the department website or elsewhere accessible. In that case you'll just make a bad impression and waste the professor's time. Also, some general questions are probably better addressed to the department secretary or to the director of graduate studies, so it really depends on the questions you have - you may be referred to these sources if your questions are ones the professor can't really answer. 

[Just Jeff]

Yes, as long as the questions you want to ask have not already been answered on the department website or elsewhere accessible. In that case you'll just make a bad impression and waste the professor's time. Also, some general questions are probably better addressed to the department secretary or to the director of graduate studies, so it really depends on the questions you have - you may be referred to these sources if your questions are ones the professor can't really answer. 

I just worried because on the application, it asks me if I spoke to someone in the department. I didn't know if that was crucial or not.

[fuzzylogician]

That depends on the field (and I don't know which field you are in). In some fields, acceptance is contingent upon there being a professor who is willing to fund you - normally in lab-based sciences. This professor will be your main advisor, will fund you, and to a large extent determine the kinds of research you can do (often you'll be working on a project that they received some funding for). In other fields, you get accepted to the department as a whole and can later decide who you want to work with. Here too, it helps to have people who you've talked to because you do need to have an advisor at the end of the day and it's a good way to demonstrate fit, but it's less crucial. Many humanities and some social sciences are like this. FWIW, I never contacted any professor prior to applying, and I think my colleagues didn't either, and we obviously all did just fine. 

[Just Jeff]

Aww, thank you so much! I'm in English btw.