How to get all category from databse

i am a android developer and my php is not good.my android project need to fetch database every day.
i have a category like follow in my site.
category:
1.console
2.tablet
3.camera
4.phone
5.laptop
6.other
my problem: structure wordpress database is very confuser.i find a my category in table wp_term but i cant find a relationship for fetch categoty.

i just want know how to get all category with class wpdb in php.

any advice is helpful.