Guys, it's like this:
You have barley. There are two main types of barley used in brewing, two-row and six-row. Two-row barley has larger, fatter kernels that grow in two rows on the stalk. Six-row barley has smaller, tougher kernels that grown in six rows on the stalk. Of the two, two-row is much more popular because it's more efficient. We'll put six-row aside for now.
There are many cultivars of two-row barley, and many of those have been mentioned in the thread. Cultivars are like dogs, onions, people, or yeast strains. They are all of the same species, but they vary for one reason or another. Marris Otter is a cultivar of two-row barley, and a very popular one in the U.K.
Now, you get to malting. There are many types of malt, labelled according to what the maltster did to the grain. Among those malts out there is "Munich malt," "crystal malt," chocolate malt," and of course, "pale malt." U.S. and U.K. maltsters have different malting techniques. U.S. maltsters tend to use U.S. cultivars and kiln their pale malt to around 2-2.5L, while U.K. maltsters tend to use U.K. cultivars and kiln to around 3-3.5L. In any case, "pale malt" always is two-row malt.
For some reason, many brewers starting referring to U.S. two-row pale malt as just "U.S. 2-row" or "domestinc two-row" or something like it. Since Marris Otter is so popular for U.K. pale malt, many brewers (through laziness or ignorance) refer to any U.K. pale malt as "Marris Otter." When I mention "Marris Otter," I mean that cultivar.
There ya go, I think.