VLOOKUP(Lookup Value, Table Array, Column Num, True/False)
Lookup Value: the value in which you are searching for
-- this can be a 'hard value' that you simply type in or it could be a cell reference if the value changes or already exists in a cell.
Table Array: the cell range in which the function is to start looking for the specified 'Lookup Value'
-- KEY: the first column in the cell range MUST contain the value that you are looking for. Ex: if you are looking for a first name, the Table Array cell range cannot start with a column containing phone numbers (it can, but the formula will never work)
-- KEY: This function will only look in the first column specified in the Table Array (which is why the above KEY point is imparative.
Column Index Number: which column in the Table Array contains the value in which you are looking for. if you Table array is A1:E10 and you are wanting the value from Col.D, you would put 4 (A=1,B=2,C=3, etc.) HOWEVER: if you specify the Table Array as C1:F10; C=1, D=2, E=3, etc...
True/False ('Range Lookup'): True will search for an approximate match to the Lookup Value and FALSE will only search for an exact match to the Lookup Value.
-- KEY: False is most common. If an exact match can not be found, the function will return the #N/A error. Either the value doesnt exist, the table array is not correct, or you could have some 'hidden' characters (like spaces) that you can see.
SAMPLES:
| Col. A | Col. B | Col. C | Col. D |
1| Name | ID | Dept. | Phone |
2| Bob | 1234 | HR | 123-1234 |
3| Luke | 0987 | Security | 123-5467 |
=VLOOKUP(E1,A1:D3,2,False)
In E1 we have typed "Luke". The formula will look at the value of E1, see "LUKE" and then go to the first column in the Table Array (A1:D3) and look down vertically for "LUKE". once it finds it, it will go to the Col. # (we had 2) and return the ID# for the Record containing "LUKE" in the first column. If we were to type "BOB" in E1, it would then change to return Bob's ID#.
If we changed the formula to have a different Col.#, say 4, it would return the Phone Number for that person.
If you type in Jason in E1, it would return #N/A because it will not find an exact match in the Table Array.
Check the following link:
http://office.microsoft.com/en-us/excel-help/vlookup-HP005209335.aspx
As Anand said, the INDEX + MATCH combination could be more helpful depending on the layout of your data.
INDEX(ARRAY,ROW#,COL#) to use these together you would place the MATCH Funciton in place of the ROW# argument: INDEX(ARRAY,MATCH(LOOKUP VALUE,LOOKUP ARRAY,MATCH TYPE),COL#)