Based on our experience, we compared Yammer and Microsoft Teams as follow,
Microsoft Teams: When a file is uploaded to Microsoft Teams, you can view/edit files on your File Explorer using One Drive for Business, similar to the way that you view/edit files on OneDrive. However to view files on on File Explorer, a business OneDrive (Office Business Essential Package, $5/month) is required. Otherwise you can edit/view files on both Microsoft Teams (Files tab) and SharePoint.
Yammer: Any file uploaded to Yammer is only accessible thru Yammer. However, Microsoft announced they are updating Yammer. Maybe in the next updates, files on yammer can be viewed/edited similar to Microsoft Teams.
Cost Per User
Teams: Each user’s cost is $5 (Business Essential Package, $5/month). However, Microsoft announced they will allow external users which would be free.
Yammer: No licence is required for users.
Planner (Task Management)
Teams: includes Planner which is like Trello, used for task management. You can create multiple columns such as “To Do” and “Done” columns. To create a task, you can add a card to a column and assign it to someone and set a deadline. Once the task is done, it can be moved to another column like “Done” column.
Yammer: doesn’t have task management.
Rich Text Formatting
Teams: You can use rich text formatting like using bold, italic and colorful texts.
Yammer: You can’t use rich text format like bold and italic.
Teams: You can’t use tags like a blog or hashtags. But you can use Teams/Channel name as tags. To use a channel/team name in a conversation, type “@” and a few letters, then some suggestions pops up.
Yammer: You can tag the post like a blog. In Yammer, tags are called topics. You can also view the list of all the posts under each tag/topic.
Teams: is made for chatting like WhatsApp or text messaging. You can chat directly with another person. Or you could create teams and share the communication among the members of the team.
Yammer: is more like Facebook in which you add a post and other users can comment.
Sharing and Linking
Teams: You can’t share the link of conversations because they don’t have a link or address. Sharing links are useful for example to direct your team member to one of your previous conversations.
Yammer: You can share the links of posts similar to the way that Facebook posts are shared.
Teams: You can edit conversations after you submit them. After a few hours, editing option is disabled.
Yammer: No editing option. You need to delete the post and re-post it.