California king sheets are sized to fit a California king mattress. Commonly found on the West Coast (hence the name), the California king is the longest standard mattress size. Standard king-size fitted sheets, therefore, are unlikely to fit a California king mattress. Other than the size, there are no differences in the sheets themselves in terms of materials or stitch-count.
There are two kinds of king-size mattresses, each with two names. A standard king, also known as an Eastern king, is the widest of the standard mattress sizes. The California king, or Western king, is the longest standard mattress size. California king sheets will fit a Western king, but might or might not fit an Eastern king.
The dimensions of an Eastern king mattress are 76 inches wide by 80 inches long. Because 78 inches is only 6 1/2 feet, individuals taller than six feet might be uncomfortable on a standard king-size mattress. The California king, at 72 inches wide by 84 inches long, is 4 inches narrow but 4 inches longer. A full 7 feet in length, the California king is the mattress of choice for tall individuals.
Fitted sheets are specially tailored for a particular mattress size. The 4-inch difference in both dimensions means that fitted sheets will not interchangeable on king and California king mattresses. Some companies, however, produce a king-size flat sheet that simply incorporates the largest dimension of each variation. The result is a single sheet that can fit both Easter and Western king mattresses.
Just like sheets, comforters are made for both Eastern and Western king mattresses. There's a bit more flexibility when it comes to comforters, though, since they do not necessarily need to fully cover and wrap underneath the mattress. A standard king comforter, though it might not fit entirely as intended, will fit on a California king bed. Conversely, a California king comforter will cover the surface of a standard king-size mattress, but will not have the width to drape over the sides to the same extent as an Eastern king comforter. Note that the designations "king" and "California king" do not denote the depth of the mattress, which can vary.
Despite the name, California king sheets can be found anywhere in the country. This is especially true due to online bedding retailers that ship across the nation and the "one-size-fits-all" flat king sheets. The geographic feature of the name simply reflects the fact that the California king mattress originated in the Golden State and has been most prevalent there.