pretty much. A string lasts from the first single or double quote to the next matching one, so your code was rendering
document.getElementById("Difficulty").innerHTML="<button id = 1 onClick="
as the first string, then junk in between, then
">Easy</button> <button id = 2 onClick="
as a separate, second string, etc
you can get around it in your case by using singles within doubles (or vice versa, not important). If you need another set of quotes in there (for example if you were trying to pass another string as an argument in your onclicks) you would have to escape the quotes, which is another way to get around the problem you were having