A literal is a value that is hard-coded into a source file. These are, more often than not, initial values of variables.
A null value can have only one value, and represents that a slot in memory has no usable value.
A Boolean value represents one of two values.
A number is a representation of any real number.
A character represents any one of the 65535 UTF-8 characters.
"String" is the term used for an array of characters representing plaintext, such as a word or sentence.