How to Customize in Office 365
https://support.office.com/en-us/article/Plan-customizations-solutions-and-apps-for-SharePoint-Online-b7898ebf-69b7-4196-81e3-b04e1a4e7d67?ui=en-US&rs=en-US&ad=US http://technet.microsoft.com/en-us/library/sharepoint-online-developer-service-description.aspx office 365 developer reference http://msdn.microsoft.com/en-us/office/aa905340.aspx