$\mathcal{U}(\mathfrak{h})$-free modules and weight representations
The classification of simple modules for a simple Lie algebra $\mathfrak{g}$ seems beyond reach: it is complete only for $\mathfrak{sl}(2)$. However, some classes of simple $\mathfrak{g}$-modules are well understood, such the category of weight modules with finite dimensional weight spaces. Irreducible weight representations were classified due to the effort of S. Fernando and O. Mathieu.