When I get a corned beef sandwich at a restaurant usually the meat has a grain and is kind of stringy. However, when I buy corned beef at a supermarket it smooth and looks similar to roast beef. What is the difference?
In addition to the direction the corned beef was sliced, you need to boil the corned beef just below a hard simmer -- not on a hard boil. I have relatives who crank the knob to 11 and boil the thing into "stringy meat toothpicks", and that's no matter which way you cut it. I cook a 3 pound corned beef in 1 gallon of water on just a hard simmer for about 4 hours and get fantastic results.
This may be due to the direction in which the corned beef was sliced. In order for it to look smooth, it must be sliced against the grain. If you want stringy corned beef ( sometimes called "pulled corned beef"), you cut it along the grain. You can tell which way the grain is by the lines on the top of the whole piece of meat.
A good restaurant's "corned beef" started out as a big piece of meat, was salted and cured for a while, then was cooked and sliced on a slicer. A supermarket's "corned beef" might be that, or it might be a bunch of miscellaneous beef pieces left over from butchering, which are mixed with salt, curing agents, and as much water as allowed by law, formed into loaves and cooked, then sliced. Since they're made from small pieces of beef, there's very little grain or stringiness.
If you want good corned beef from a supermarket, go to the deli counter and make sure they're cutting it from something that looks like it came from an animal. (There are brands of prepackaged corned beef that aren't awful, but they can be difficult to find.)