If you only need the most basic capabilites of Office for iPad, the free version that is available for download from Apple's app store may be just fine for you.
But if you want to do anything like editing, formatting, saving to OneDrive or SharePoint, you need an Office 365 subscription.
Here's Microsoft's chart that shows what you get for free and what you don't:
If you need the paid version, the following Office 365 plans include Office for iPad support: Office 365 Home Premium, Office 365 Small Business Premium, Office 365 Midsize Business, Office 365 E3 and E4 (Enterprise and Government), Office 365 Education A3 and A4, Office 365 ProPlus, and Office 365 University. Once it's available, the just-announced Office 365 Personal plan will also qualify.
Update: Speaking of paid vs. free, Microsoft also announced yesterday that it is no longer requiring iPhone and Android phone users who want to use Office Mobile (the phone-specific version of Office) to have an Office 365 subscription. Both versions are now truly free.