Microsoft have released the much awaited version of Office for the iPad, consisting of Word, Excel, PowerPoint and, er, OneNote. It is based on Office 2013, modified to make it easier to use in a touchscreen environment. However, the menus have been somewhat simplified.
It is available to download from Apple's App Store, although note that each program has to be downloaded separately. It is free of charge - sort of. That is, the programs will allow you to view documents, spreadsheets and presentations, but not make any changes to them or create new ones. To do this, you need to have an Office 365 subscription (see here for details), at which point all the features become available. It is not possible to purchase Office for iPad in any other way. Everything is tied into OneDrive (formerly known as SkyDrive), so this is where the files have to be stored.
For people who do not have or want Office 365, the web versions of Word, Excel and PowerPoint are available by signing into OneDrive. Interestingly, at the time of writing they seem to offer more capabilities that the official Office for iPad...