An Excel function is a pre-built formula with a specific purpose. Excel provides hundreds of functions in various categories like dates and times, text, statistics, lookup, financial, engineering, etc.
Most Excel functions require specific inputs, called function arguments. For example, the DATE function creates a date value based on three arguments: year, month, and day:
=DATE(year,month,day)
Some functions don't require any arguments. For example, the TODAY function returns the current date without any input:
=TODAY()
Formulas with multiple functions
=A1+A2
Formulas can include multiple functions and/or constants. For example, the following formula returns a date 30 days from today:
=TODAY()+30
The formula below uses three different functions to calculate current age based on a birthdate in A1:
=INT(YEARFRAC(A1,TODAY()))
See this page for a full explanation of this formula.
Formulas vs Functions
When you use a function in Excel, you are using a formula. So, by definition, all functions are formulas. However, you can create formulas in Excel without any functions at all. For example, this formula returns the sum of A1 and A2 without any functions.
