Cluster HeatMaps

I had heard of HeatMaps, but heard of Cluster HeatMaps very recently only when I had to generate one. It is being used a lot in Bioinformatics for gene expression studies. 

Weinstein describes HeatMaps as following: (taken from here)

"In the case of gene expression data, the color assigned to a point in the heat map grid indicates how much of a particular RNA or protein is expressed in a given sample. The gene expression level is generally indicated by red for high expression and either green or blue for low expression. Coherent patterns (patches) of color are generated by hierarchical clustering on both horizontal and vertical axes to bring like together with like. Cluster relationships are indicated by tree-like structures adjacent to the heat map, and the patches of color may indicate functional relationships among genes and samples. "

I could not, for long  time figure out how exactly they are generated. Reading here I got to understand that Cluster HeatMaps are generated by first performing hierarchial clustering for rows (i.e.genes) and then for columns(i.e. samples), generate their dendrograms and then use those for generating heat maps with trees for both genes and samples. I got to learn to do this in R(using hclust, heatmap functions) looking up here.(nice tutorial on R & Bioconductor)