Just found this on the IRS website, thought this would be a good resource to start with in the USA. Others can post for their country, if there are any.
there are many great reasons to run a business, tax wise, but the IRS is the final say if it'a business or hobby, and therefore what's a legitimate deduction and what's, umm, tax fraud. more info here - //www.nysscpa.org/cpajournal/2004/904/essentials/p46.htm
