The main mathematical topics in 8th grade (taken from the Common Core Standards) are:

(1) Formulating and reasoning about expressions and equations, including modeling and association in bivariate data with a linear equation, and solving linear equations and systems of linear equations

(2) grasping the concept of a function and using functions to describe quantitative relationships

(3) analyzing two- and three-dimensional space and figures using distance, angle, similarity, and congruence, and understanding and applying the Pythagorean Therorem.

* These are general overviews.  For more specific standards, follow the "Common Core Learning Standards" link on the left.