An expression which calculates values of cell.

Types of function:
Count and sum.
Logical.
Cell reference.
Date and time.
Text.
Lookup and reference.
Financial.
Statistical.
Round.
Formulae errors.
Array formula.


Count:
To count the no of cells that contain functions.

Countif:

Countifs: 

CountA:
Not only read no's but also characters.

Logical:
IF:

AND:

OR: 

Cell reference:
Relative reference:

Date and time functions:
DATE:

DATE with TIME:
YEAR:
Round:
Roundup:
Rounddown: