Yes, it is income. Whether you choose to report it as such is your decision. I suggest consulting with your tax professional.
I know that rental properties are considered rent so I would assume the same principle applies here. If it is your house, then a person renting a room would be considered a source of income. I don't think the relationship of the person to you would make a difference. The only case in which the relationship might make a difference that I can think of is in the case of a minor. A dependent child living at home might not necessarily be considered income even if that child paid rent. For instance, a 20 year old who has moved back home but is still claimed on your taxes as a dependent (I think children can be claimed until age 21 but that might have changed). It might not be considered income if the person is simply paying for their extra portion of utilities or groceries. Since the question asked about rent specifically, I am assuming that the person is paying more than just a share of extra expenses and is paying for the use of the space. This would likely be considered income.
I believe if you own the house and are renting (recieving moneys for the service of living there) that is income, regardless of the living arrangements or who the person is. IF you own the space and the other person is paying to live there, it is income. Whether you choose to claim it, is your choice. Many, many people do not. However, legally, as mentioned above any income recieved from a single source that totals to over $600 in a year should be claimed on your taxes.
I probably should have mentioned that it is not really a roommate. It is a family member sharing the house with me. All areas are common areas, basically. Everyone has a room, but everyone can go in any room.
I will preface this answer by saying I am not an accountant or a tax attorney. With that being said, anything you bring in is technically considered income. I have run several small businesses and my understanding is that you are only required to claim income from someone who you receive more than $600 from in return for your services or products. More than likely you will take in more than $600 from your roommate, so I would say you will have to claim that income.
Again, I would consult a professional for a definitive answer, but I'm pretty sure you will have to claim that if you want to abide by the letter of the law.
You would probably have a pretty easy time concealing this income if you fail to report it. But it is still income. It doesn't really matter that it seems informal to you. It is money that you are getting in return for letting someone live in your house so it is income.
what if you dont rent the room out and let them stay for free, but they pay halfo of the bills? Or what if its a girlfriend just paying her share of the bills?