You can purchase Office [Year] as a standalone program or purchase a subscription service to Office 365.
Office 2016 (current edition) will run $149.99 and licenses one user on one PC. It includes Word, Excel, PowerPoint, and OneNote.
Office 365 Home or Personal will include Word, Excel, PowerPoint, OneNote, Outlook, Publisher, and Access. It also includes 1TB of OneDrive cloud storage space and 60 minutes per month of Skype calls per user. This will run $99.99/year (or $9.99/month) for Home and $69.99/year (or $6.99/month) for Personal. Personal is for 1 user only. Home is for up to 5 users. It also includes apps for devices (including Apple products).
When you have Office 365, you also get the latest Office software. It also has automatic billing so you won’t ever run out. With Office 365, you also get support for any issues.
When Office 2019 is available later this year, it will require you to have Windows 10.