hangout by google is an attempt from google to give tough competition to other rival messaging app like whatsapp,line,hike,we chat,skype,viber etc.
the app is available for download for free from google play store and it is pretty much big in size for around 25mb. the app is automatically downloaded by all android phones and you cannot remove it from your phone,so its better that you start using it.
you can message an video chat for free with your google contacts using this app.you need not their mobile phone number unlike whatsapp ,with the email id you can text them as well as do a free video chat with them.
the interface of the app is pretty much simple and easy to get used to,the app also acts a default messaging app so all your text messages can be handled by it also.
the video call quality is pretty much good and it provides hd video chat too.but you need a very good net connection for that its not like whatsapp or hike which are optimized for 2g connection to provide a seamless chat experience.
but the main benefit you have with this app is the video call functionality and group video chat feature which is popularly called as hangout.