Android 6.0 marshmallow exists on a multitude of devices, with more carriers & devices receiving updates in the close to future. The biggest features of this version are little things individuals can appreciate on a daily basis, like individual app permissions, native support for fingerprint sensors, and granular context features for Google currently. We're also seeing developers do additional with Chrome, better app-to-app communication, and new power management features.
The big question is:When can Android owners download and start using it?If you have a nexus 5,6,7(2013),9,player or Android One,the answer is easy.Google is now sending out Marshmallow in stages,and its depending on your carrier,you have access to it now or will shortly.
If you ordered the new Nexus 5X and 6P phones,Marshmallow will come installed on those devices. But Nexus devices make up a tiny part of the Android market.so if you own a Samsung,LG,HTC,Sony or Motorola, you need to wait till your handset maker and mobile carrier prepare the update for you.