A couple of months back, the Azure Active Directory team began rolling out a new login experience for Azure AD, and any application that utilizes Azure AD (such as Office 365). You likely did not notice this unless you happened to click the box in the bottom right corner of the page indicating that you wanted to try out the new login experience. Fast forward to today, the new login experience is rolling out to all tenants and you will be seeing it very soon. This has the potential to cause a panic with users, as security awareness training often encourages them to be wary of changes in appearance to websites they use on a regular basis. Fear not! This is an intended change and you can read the official statement from Microsoft here.
The OLD sign in experience:
The NEW sign-in experience: