Excel offers a vast library of functions categorized to perform various tasks. Here's an overview of some common types and how to use them:

1. Mathematical and Trigonometric Functions:

SUM: This calculates the total of a range of numbers. For example, `=SUM(A1:A10)` adds values from cells A1 to A10.
AVERAGE: This calculates the average of a range of numbers. For example, `=AVERAGE(B2:B15)` finds the average of cells B2 to B15.
COUNT: This counts the number of cells containing numbers in a range. For example, `=COUNT(C1:C20)` counts numeric entries in cells C1 to C20.
MIN and MAX: These return the smallest and largest values in a range, respectively. For example, `=MIN(D3:D8)` finds the minimum value from D3 to D8.
SQRT: This calculates the square root of a number. For example, `=SQRT(E5)` finds the square root of the value in cell E5.
SIN, COS, TAN: These perform trigonometric calculations for sine, cosine, and tangent of an angle in radians.

2. Text Functions:

CONCATENATE: This joins multiple text strings into one. For example, `=CONCATENATE("First Name: ", A2)` combines "First Name: " with the content of cell A2.
LEFT, RIGHT, MID: These extract specific parts of a text string. For example, `=LEFT(B3, 3)` extracts the first 3 characters from cell B3.
UPPER, LOWER: These convert text to uppercase or lowercase, respectively. For example, `=UPPER(C1)` converts the text in cell C1 to uppercase.
LEN: This returns the length (number of characters) of a text string. For example, `=LEN(D7)` finds the length of the text in cell D7.

3. Logical Functions:

IF: This allows you to make conditional decisions. For example, `=IF(E4>100, "Over Limit", "OK")` displays "Over Limit" if the value in E4 is greater than 100, otherwise it displays "OK".
AND, OR: These combine logical conditions. For example, `=AND(F2>5, F2<15)` is TRUE only if the value in F2 is both greater than 5 and less than 15.
NOT: This reverses a logical condition. For example, `=NOT(G1<0)` is TRUE if the value in G1 is not less than 0 (i.e., zero or positive).

4. Lookup and Reference Functions:

VLOOKUP: This searches for a specific value in a leftmost column of a table and returns a corresponding value from another column within the same table.
HLOOKUP: Similar to VLOOKUP, but searches a top row instead of a leftmost column.
INDEX MATCH: This is a powerful combination that offers more flexibility than VLOOKUP/HLOOKUP.

5. Date and Time Functions:

TODAY: This returns the current date. For example, `=TODAY()` will always display the current date when you enter the formula.
NOW: This returns the current date and time. For example, `=NOW()` will show the exact date and time you entered the formula.
YEAR, MONTH, DAY: These extract specific components from a date value. For example, `=YEAR(A1)` returns the year from the date in cell A1.
DATE: This allows you to create a date value by combining year, month, and day numbers.

Using Functions:

1. Click the cell where you want the result to appear.
2. Type the equal sign (=) to indicate you're entering a formula.
3. Type the function name (e.g., SUM, AVERAGE, etc.).
4. Open parenthesis ().
5. Select the cell(s) or range you want to operate on. You can also type numbers directly within the parenthesis.
6. Close parenthesis ().
7. Press Enter. Excel will calculate the formula and display the result in the chosen cell.

Tips:

Excel offers helpful auto-complete suggestions as you type function names.
You can use cell references within functions to make them dynamic and adaptable.
Explore the "Insert Function" button (fx) on the Formula