The Outlook web app brings your email and calendar into a browser. Access your emails, contacts and calendars from anywhere. Create, edit, and add meetings, appointments, and other events to your calendar.
Teams is a collaboration workspace that helps you engage with your team members via chat, online meetings, and file sharing. Several of the Office 365 Apps can be added as a tab into your Team space, so you can use Teams as your work hub, rarely needing to navigate away when working with your co-workers.
Skype for Business is a communication and collaboration tool. You can talk to others via the messaging functions and also schedule online conference calls and meetings.