Android App integration with other websites

Android App integration with other websites

I am currently a student of Android. My aim is to develop Android Apps
that talk with different websites and exchange/process/return data. But I
am really not sure what should be starting point.
Coming from a backend development I can put the layout for backend
development as follows
a) Understand Relational DB concepts
b) SQL
c) PLSQL
d) Procedures/Functions
e) Solve realworld problems ....etc
Similarly can someone tell me what should be starting point for learning
Android integration.
HTTP or Web Services or REST API or JSON/XML or anything else
All these terms are new to me and when I googled/wikied the terms I can
(kinda) grasp what they mean but I am not able to see the big picture as
to how they fit etc...wrt integrating an app with a website.
TLDR.what should be the starting point for learning Android Integration ?.
PS : If this is a wrong forum, then do let me know which forum to post
this on. Thanks !!!