Microsoft Teams

Designed to compete against similar enterprise-oriented apps/services such as HipChat, Hangouts Chat, not to mention, Slack, Microsoft Teams is a collaboration and chat app mainly aimed at businesses.

Built around some of Microsoft's best services

Since it's a business-only app, it requires an Office 365 Business Premium subscription in order to get started with it. It's available in 181 countries and comes with support for 19 languages, and it features clients for both desktop (Windows and macOS) and mobile platforms (iOS and Android).
The best thing about Microsoft Teams is the fact that it perfectly integrates with a broad range of other useful Microsoft services. For example, it comes with support for all Office-related files, for Skype video and audio calls, OneDrive, Power BI and Planner, and so forth.

Modern-looking, user-friendly and very well-documented collaboration app

Microsoft Teams provides its users with a very functional and business-like collaboration environment. Microsoft has kept a close eye on the balance between features and user-accessibility.
Sure, the app packs a plethora of useful features, but the most impressive thing is that you don't really feel assaulted by these features. All the tools and features are neatly organized and are always within easy reach, without ever getting in your way. It's worth pointing out that you are provided with a comprehensive (and quite interactive) documentation section, courtesy of T-Bot.

Powerful app with a strong focus on collaboration and accessibility

There are five, self-explanatory sections: Activity, Chat, Teams, Meetings, and Files, each and one of which is neatly organized with the help of tabs. Everything you would expect from a collaboration and chat application is here. Team conversations can be started and managed with ease, as well as be organized by topic, project, or with the help of other parameters.
Files, websites, notes, and chats can be reached by any team member. The Activity section acts as your personal "news feed" and is the place where you can view all your likes, mentions, and other types of contributions. You can also keep an eye on your schedule for the day and even thoroughly schedule meetings, as well as conveniently access all your files from your OneDrive Business account.

Microsoft's response to Slack and HipChat is well worth your attention

Taking everything into account, we can safely say that Microsoft has done a great job with this first public release of Microsoft Teams. Undoubtedly it's an excellent solution for business chat and collaboration, packing useful tools, bots, and sublime Office and Skype integrations.
If, at first, you might be tempted to think that since it's so robustly built around Office 365, Microsoft Teams does not offer a wider degree of "integration" freedom (like in the case of HipChat and Slack), think about it this way. Office 365 has a massively overwhelming number of active users (at least ten times more than any collaboration app), all of which are used with most of Microsoft's services and all of which have now gained the possibility to use a bespoke collaboration app, without having to pay something extra for it.