What is Conflict of interest?
“Conflict of interest is a set of conditions in which professional judgement concerning a primary interest (such as patients’ welfare or the validity of research) tends to be unduly influenced by a secondary interest (such as financial gain).” 
What do you need to declare?
It is better to “err on the side of greatest disclosure”  when declaring conflicts of interest. In other words, it is best to disclose all competing interests that would embarrass you if it came under scrutiny after publication .
Examples of conflict of interest include the following relationships with firms that have a stake in the subject of the manuscript:
Financial – any financial relationship e.g. grants and research funding; reimbursement for attending a symposium; a fee for speaking; a fee for consulting; travel grants etc.
Employment – direct employment (full- or part-time). This includes current negotiations regarding future employment.
Stock ownership – any stock or investment in the firm
Intellectual property – any patents, royalties etc.
Personal – a close relationship, academic link, or rivalry with a person, political party, or special interest group whose interests may be affected by publication of your paper [2–4].
It is important to note that declaring potential conflicts of interest does not necessarily imply any wrongdoing. It demonstrates good publishing practice; and upfront declarations will increase readers’ confidence in your work by allowing them to independently assess any potential bias . Thompson DF. (1993) Understanding financial conflicts of interest. The New England Journal of Medicine; 329: 573–576  Goozner, M., Caplan, A., Moreno, J., Kramer, B.S., Babor, T.F. and Husser, W.C. (2008) A common standard for conflict of interest disclosure. Center for Science in the Public Interest. Available at: http://www. cspinet.org/new/pdf/20080711_a_common_standard_for_conflict_of_interest_disclosure__final_for_conference. pdf [Accessed 11 April, 2016].  BMJ Publishing Group Ltd. (2016) BMJ Declaration of Competing Interest. Available at: http://www.bmj.com/content/suppl/2003/03/13/317.7154.291.DC1 [Accessed 11 April, 2016].  Edanz Group Global Ltd. (2016) Declaring Conflicts of Interest. Available at: https://www.edanzediting.com/resources/declaring-conflicts-interest [Accessed 11 April, 2016].