# 11. Core modules¶

The MDAnalysis.core modules contain functionality essential for MDAnalysis, such as the central data structures in MDAnalysis.core.universe and MDAnalysis.core.groups or the selection definitions and parsing in MDAnalysis.core.selection.

## 11.2. Important objects for users¶

All users of MDAnalysis need to understand the two most important classes in this section, namely the Universe and the AtomGroup.

## 11.3. Topology system¶

The topology system is primarily of interest to developers.

## 11.4. Selection system¶

The selection system is primarily of interest to developers.