ArcGIS Pro is an application for creating and working with spatial data on your desktop. It provides tools to visualize, analyze, compile, and share your data, in both 2D and 3D environment.
ArcGIS Pro can be installed side by side with ArcGIS (ArcMap).

You can use an ArcGIS online account to publish maps to the web. More information on how to login and create an account:
https://software.itc.utwente.nl/public/software/arcgisonline.aspx

The following ArcMap tools are not available in ArcGIS Pro:

http://pro.arcgis.com/en/pro-app/tool-reference/appendices/unavailable-tools.htm

In upcoming releases tools will be added however some tools, specifically the coverage tools will not be made available in ArcGIS Pro.
Nomally there are 2 releases per year, for more information on see the ArcGIS Pro roadmap:

https://geonet.esri.com/community/gis/applications/arcgis-pro/blog/2017/03/05/arcgis-pro-roadmap