Types

A workbook in Excel is a basically a 2D array of input/output cells. Each cell can have a value with one of following types

Blank - an empty value. Mostly for blanks cells or used for optional arguments of functions.
Logical - a boolean value
Number - an equivalent of double, but without NaN or Infinity. Number also represents dates/timespans through serial datetime.
Text - A text of up to 32767 characters
Error - One of the excel errors, #DIV/0

In the future, we will call a union of these types a scalar value.

Workbook also contains formulas, a receipies that take a values and calculate a new values. Formulas are used in

cells formula - in that case, the output of the formula is written into the cell
names - although names are mostly used to refere to a range of cells, they are formulas and thus can contain any formula expression (e.g. 1+2)
array formulas - formula changes value of 2D array of cells.

Values used during formula evaluation can have the types that are in cell, but in addition can have also following types: