On the 27th March 2014 Microsoft finally announced Office for iPad (iOS).
Depending on who you are this may sound like brilliant news but frankly Microsoft are late to the game and there is one big gotcha. Apple has had their own iWork software available on iOS for some time now (Pages, Numbers and Keynote) and the latest versions are compatible with their OS X counterparts plus they can handle basic Microsoft format documents (incidentally the iWork apps now come free with all new Apple hardware purchases). The big gotcha is that although the Microsoft iOS apps are free, they can only view/present documents. To be able to create and edit documents you need to pay for an Office 365 subscription!
If you have already bought and paid for Office 2011 (but not as Office 365) then you have no way to purchase and use the Office for iPad apps without paying all over again for the Office 365 subscription. Maybe at some point Microsoft will offer a new version of Office for OS X but until then, if you already have Office 2011, the Office 365 is not a great value proposition unless you really need Office for iPad and/or the online services that are included with Office 365 (OneDrive storage, Skype minutes, etc). Office 365 is offered as a 30-day trial from Microsoft's website but note that you must provide a payment method. The trial can be cancelled within the first month without being charged. Most Office 365 subscription plans include all the Office 2011 desktop applications for OS X including Outlook, Office for iPad iOS apps including OneNote, plus OneDrive (formerly known as SkyDrive) online storage space and Skype minutes. The subscription licensing is similar to Adobe's Creative Suite in the Cloud. You pay monthly/yearly, you download the software, it works while you pay, it stops working when you stop paying. If a new version comes out while you are subscribing you get the new version.