Now Odoo has given lot of opportunities to develop our knowledge, As Ray said like, odoo have the elearning platform available to learn all the basic of Odoo including the  the  functioning  of  fundamental  modules.   

Follow this link ------>  https://www.odoo.com/slides/all. You can able to enroll the courses, watch and do the sample quiz available. And you must have to try the  https://www.odoo.com/slides/odoo-certification-sample-test-50..

Thereby definitely you can understand the stuffs..

All the best !!

I also recommend to read the whole documentation for the version of Odoo you want to ger certified on.