The serial input basics methods that return multiple characters return strings (null terminated character arrays), not Strings (of the String class). It is recommenced to not use the String class with the Arduinos with small amounts of SRAM as memory problems can occur. See the evils of Strings.
That means that you cannot use equality (==) to compare strings and you cannot use + to concatenate strings. There are a set of functions to do those things with strings. See HERE.