The butter can be
sweet cream butter lightly salted or unsalted... but the label will tell you.
Also, there is more water content in American butter (butter not margarine, that's a whole another thing) than European (French / English) butter. Also, some butters, are not as good as others. Butterfat level can differ slightly by different manufacturers.
Irish butter is good too.
Don't assume that the european brands have higher butterfat content.
here is some info on butter (salted vs unsalted) and other test results from Cook's Illustrated:
"There are a couple of reasons why most cookbooks and magazines, including Cooks Illustrated, call for sweet butter and salt instead of salted butter in recipes. First, some salted butters are saltier than others. This poses a problem for recipe writers and can lead to dishes with too much or too little salt. Adding salt separately makes it easier to duplicate the total amount of salt in the original recipe.
Secondly, salted butter has a different flavor than sweet butter combined with salt. Traditionally, salt was added to butter to extend shelf life. Refrigeration has made this unnecessary, but many dairies still add salt for flavor. However, we find that salt often masks the sweet creaminess associated with the finest farm-fresh butter. Especially when baking, we miss this flavor. In a blind test of sugar cookies made with salted and sweet butter, the results were quite clearly in favor of cookies made with unsalted butter.
Lastly, salted butter almost always contains more water. Water content in butter can range from 10 to 18 percent. (By law, fat content in butter must exceed 80 percent.) In baking, the butter with the lowest water content (i.e., sweet butter) is preferred, since excess water from butter can interfere with the development of gluten in the flour."
their taste tests rate Land O'Lakes Ultra Creamy butter and Ultra Creamy unsalted butter as the bes tasting
and finally re: fat content:
There's been plenty of hype about the merits of "premium" butters (often called European-style butters), touted for their creamy texture and high fat content compared with regular butter. In the past, when the test kitchen conducted blind taste tests of both styles, tasters found only subtle differences when the butters were sampled plain (simply spread on toast), and those nuances disappeared entirely when they were used in cooking. Was our panel missing something?
"It's when we hunkered down to calculate the percentages ourselves (dividing the amount of fat per serving by the 1-tablespoon serving size) that a clearer picture began to emerge. Initially, we were puzzled to find, as you did, that most manufacturers don't list fat percentage on the packaging--especially given that fat content is a key selling point. Even more surprising, the fat content of every butter we found--from the ubiquitous Land O' Lakes to the fanciest French beurre--fell between 11 grams and 12 grams per serving. Just a 1-gram difference. What's more, many of the so-called premium butters (including President, Kerrygold, Lurpak, and perhaps the best known, Plugrá) listed just 11 grams of fat per serving. How could it be that these self-proclaimed "premium" butters contained the same amount of fat as the generic (cheap) supermarket brands?
As it turns out, it all boils down to U.S. Food and Drug Administration labeling laws, which allow manufacturers to disclose fat content in grams rounded to the nearest whole number. That means a butter with "11 grams" of fat may contain between 80 percent and 82 percent fat (82 percent fat is generally considered the minimum fat content for a premium butter). No wonder our tasters noticed hardly any difference between cheap butter and premium butter in our battery of blind taste tests--the difference comes down to less than two percentage points.
If you want a higher-fat butter, choose one with 12 grams of fat per serving rather than 11--which is guaranteed to be at least a few percentage points higher in fat content. In a 2004 tasting, our tasting panel liked the rich flavor and creamy texture of Land O' Lakes "Ultra Creamy" variety (which has 12 grams of fat). "
jmt...thanks for all the info...and everyone else for responding....
Just fyi....I re-checked the label...it is Kirkland brand (Cosco) sweet cream butter. The ingredients are cream (milk), salt. There is no other mention of "salted" or "unsalted" anywhere.
It doesn't taste very salty....I have "salted butter" which tastes much saltier, but I can detect a little saltiness in it.
I just made the best buttercream I have ever made using this butter. The slight saltiness of it totally cut the sweetness of the buttercream. The texture is as smooth as silk.....I mixed on low (1 on the KA) and there is not a bubble to be seen. I have always previously used "unsalted butter" and though the results were tasty...this is much better.
I am using a 50/50 butter/Crisco mixture in case you were interested. 
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