Cortana for Android will be a standalone app. It will not offer everything that Cortana for Windows Phone is offering (since it's not deeply integrated into Android), instead acting as a companion app for your Windows PC (assuming you have one). For example, you can ask Cortana on Android to remind you to do something at 10 PM, and, if at that time you're using another device with Cortana, you'll be reminded there.
Cortana is also coming to Xbox One, and, of course, it will be integrated into Windows 10 from day one (in case you didn't know, Windows 10 will be officially available starting July 29).
We recently conducted a poll to find out how people were feeling about Cortana coming to Android and iOS. It turned out that most Android and iOS users are looking forward to trying the app. This, of course, is great news for Microsoft.
source: Microsoft via ZDNet