I get that. It certainly is among the most common, for what it's worth.
I suppose I should make use of the database here to illustrate, and invite further inspection of the relevant database entries for more images. These are the plants we are comparing, in general terms:
Note the golden barrel does not flower until it is quite big (at least a foot wide) and only when it has developed a wooly central area, and the flowers barely protrude beyond the length of the spines.
This is a classic bot garden plant, perhaps the number one large cactus, and it does get pretty spectacular in old age. A wide plant when mature, usually solitary, impressive in groups.
Echinocactus and the related Ferocactus comprise most of the barrel cacti, as such. These are globose to cylindrical plants with stout spines, at least one of which is usually ribbed. They are from North America (primarily Mexico).
On the other hand, Echinopsis (and its sister genus Trichocereus, treated as a synonym by the database) are South American plants. They are primarily cultivated for their flowers, which are large and may appear in groups. These flowers tend to have hairy buds and pericarpels, they often have long tubes (see example above), and they tend to be white (though the colored ones have been bred to yield some stunning hybrids).
