Whole Word
The Whole Word expression type is closely related to the Boolean expression type, the main difference being that a Whole Word search looks for word boundary markers for each match, e.g.
Containing Text: self AND help
will only match self and help with Whole Word but would match itself or helpful with a Boolean search, ie a Boolean search partially matches terms.
Also, remember that there is an implicit AND if you don't put anything, e.g.
Containing Text: self help
is the same as "self AND help".
Plain Text
The Plain Text expression type used to be called Literal Text and indicates that the search term entered will be matched exactly as entered. For example:
Containing Text: self AND help
will look for the text self AND help, ie the AND is not treated as an operator but simply as 'plain text'. The same goes for quotes:
Containing Text: "self help"
would match "self help" including the quote marks whereas with Whole Word the quote marks indicate that the text is a phrase and would match just self help without the quotes.
Finally, remember that text entered is literal so:
Containing Text: self help
will search for exactly self help, ie. the words must be together and separated with only a single space and therefore must be on the same line of text.