How important are informal enquiries? I have finished my BSc and I am considering applying for a Msc. I have no idea about the etiquite of applying for grad school. Do I apply using the online form or should one send an email to the proffesors and doctors?

If they are important, what does one say!?

Thanks, and any advice you have is welcome!
Although referring the concerned institution guidelines for application should be self-sufficient, it wouldn't be wrong to ask a professor of the related department for any clarifications. But just make sure you go refer the guidelines first before you send a request-for-clarifications email.