The Yammer App is an app part available for free in the SharePoint store that allows you integrate Yammer conversations into SharePoint 2013 on premise or SharePoint Online if you are using Office 365.
Microsoft has announced that the Yammer App will be discontinued and will stop working as of September 2015. If you are using the Yammer App currently, you need to replace it with new Yammer Embed feature.
Using Yammer Embed, you can provide similar integration between Yammer and SharePoint including display of my feed, group feeds, topic feeds, or specific object feeds. Instead of Yammer being an App Part, the Yammer Embed is just HTML script that you can embed anywhere in your page.