The Excel VLOOKUP function 'looks up' a given value in the lefthand column of a data array (or table), and returns the corresponding value from another column of the array.
The syntax of the function is:
where the arguments are as follows:
lookup_value    The value that you want to look up.  
table_array    The data array or table, containing the search values in the left hand columns and the values to return in another column.  
col_index_num    The column number, within the supplied table_array, that you want to return a value from.  
[range_lookup]    An optional logical argument, that describes what the function should return in the event that it does not find an exact match to the lookup_value. The [range_lookup] can be set to TRUE or FALSE, meaning:  

Wildcards
In textrelated Vlookups, where an exact match is required, the lookup_value can contain the following wildcard characters:
?  matches any single character
*  matches any sequence of characters
Columns A and B of the spreadsheet below list an inventory of grocery items and their prices. Cell E2 of the spreadsheet shows a simple example of the Vlookup function being used to look up the price of an item from the inventory.
A  B  C  D  E  

1  Item Description  Cost ($)  Current Item:  Current Item Cost ($)  
2  Tinned Tomatoes  $0.90  Cornflakes  =VLOOKUP( D2, A:B, 2, FALSE )   returns the value $3.50  
3  Tinned Tuna  $1.50  
4  Cornflakes  $3.50  
5  Shortcake Biscuits  $1.00  
6  Toothpaste  $4.10  
7  Tinned Baked Beans  $0.99  
8  White Sliced Bread  $0.80  
9  . . .  . . . 
The above Vlookup function returns the price for "Cornflakes", which is $3.50.
In this example:
Columns AC of the spreadsheet below list the grades that are assigned to examination marks lying within the ranges 044%, 45%54%, etc.
Cell F2 shows the score of 52% that was achieved by the student "Anne" in an examination. The Vlookup function in cell G2 looks up this score in column A of the spreadsheet and returns the associated grade from column C.
Note that, in this example, if the exact score of 52% is not found in column A, we want, instead, to use the nearest value below this score.
A  B  C  D  E  F  G  

1  Lower  Upper  Grade  Name  Score  Grade  
2  0%  44%  F  Anne  52%  =VLOOKUP( F2, A2:C7, 3, TRUE )   returns the value "E"  
3  45%  54%  E  
4  55%  64%  D  
5  65%  74%  C  
6  75%  84%  B  
7  85%  100%  A 
The above Vlookup function returns the grade for the score 52%, which is E.
In this example:
For a practical example of the Vlookup function being used to create a variable chart, see the Excel Variable Chart page.
Also, there are several further Vlookup examples on the Microsoft Office website.
If you get a Vlookup error this is likely to be one of the following:
#N/A    Occurs if the Vlookup function fails to find a match to the supplied lookup_value. The cause of this will generally depend on the supplied [range_lookup] argument:  
If you still can't understand why you are getting this Vlookup error, check out the Vlookup #N/A Error page.  
#REF!    Occurs if either:
If you need further help with this Vlookup error, check out the Vlookup #REF! Error page.  
#VALUE!    Occurs if either:
 
For further Vlookup troubleshooting tips, there is an excellent Quick Reference Card available on the Microsoft Office website 