The document describes and compares different hierarchical clustering algorithms:
1) Single-link clustering connects clusters based on the closest pair of patterns, forming elongated clusters. Complete-link connects based on the furthest pair, forming more compact clusters.
2) Complete-link is more useful than single-link for most applications as it produces more interpretable hierarchies. However, single-link can extract certain cluster types that complete-link cannot, like concentric clusters.
3) Average group linkage connects clusters based on the average distance between all pairs of patterns in the two clusters. It provides a balance between single and complete link.