Android 5.1 (Lollipop) was officially announced on March 9th, 2015.
You can review the high-level changes at the announcement on the official Android blog.
Of course, not all devices will get this update right away and others won't get it at all. Each manufacturer and carrier often choose to add their custom modifications which tend to delay the official release.
When will each device get Android 5.1, either officially or unofficially, if ever?