I am trying to learn about the 'Hom-description' of the class group $Cl(A)$ of an $R_K$-order $A$ in $K[G]$ where $K$ is a number field with ring of integers $R_K$ and $G$ is a finite group.
I've tried Googling the term but I haven't yet found any readable sources.
Are there any good explanations of what the Hom description is available online?
Many thanks in advance.