You can purchase Office 2013 still. However, Office 2016 is coming out this fall.
The difference between Office 2013 and Office 365 is that Office 365 IS Office 2013, except that it has been continually being improved and you get updates AS Microsoft makes improvements instead of having to wait for the next service pack. Also, you get the "latest" version of Microsoft Office for the duration of your subscription. That means if you subscribe for one year now, you will install the latest Office 2013 with all the updates and then install Office 2016 as it becomes available.
Office 2013, Office 2010, and the upcoming Office 2016 are "purchase once" types of items. You buy it, and your license allows you to keep it forever. You pay a lot more up front, and when the product reaches end of service, like XP recently did, then you will no longer get support or security fixes.
Office 365 is a subscription service. You either pay a monthly fee or a discounted annual fee. You can get a subscription for a month at a time, a year at a time, all the way up to 5 years I believe. Once the subscription is ended, you either renew and continue paying, or you have to uninstall and stop using the product.
The subscription service does guarantee that you will always receive support for as long as you have your subscription. You will always have the latest version of Office with the latest updates for that version. You will always receive the hotfixes, bug fixes, security updates, etc.
Personally, I decided to go with the 365 subscription, as my family uses MS Office quite heavily.
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