Using Excel Formula Bar ...

Excel has a large variety of embedded equations to help make your life easier when it comes to spreadsheet functions. The list of Excel functions is too large to place here. However, you can click the fx button on the formula bar to list all the functions available.

Functions are generally named relative to what they do, are in all caps, and follow this format:
=SUM(A2:A8) = alerts Excel to the point of a function, SUM (or whatever) is the function call, and the paranthesis list off the argument list, which could be numbers, cell addresses, or even other functions. The function example, =SUM(A2:A8), adds cells A2-A8 together.

Functions include most mathematical functions. From Trignometry (SIN, COS, TAN, ATAN, etc.) to logical functions (AND, OR, work on checking boolean values of a set of cells) to regression curves and placing today's date (TODAY()) on top of a spreadsheet. Asking for a list of function by category in Excel Help is probably the easiest way to search through the system.

Common formulas include

=SUM(cell:cell) or =SUM(number1, number2, etc) up to 30. Sums a bank of numbers.
=exp(x) returns e to the x power.
=ABS(number or cell) returns absolute value.
=LOG(number,base) returns logarithm to an (optional) base (default = 10)
=LCM(number1, number2, etc.) returns the least common multiple of a list of up to 30 numbers.
=AVERAGE(cell:cell) returns all values in a range.
=AVERAGE(cell:cell, number1, etc.) returns values in a cell block range, plus extra numbers.
=VAR(cell:cell) Estimates sample variance.
=TODAY() Returns a serial number saying how many days since 1/1/1900
=YEAR(serial number) =DAY(Serial Number) Converts serial numbers to years or days.