Returns a value by looking up a related value on a custom object similar to the VLOOKUP() Excel function.

The field_to_return
must be an auto number, roll-up summary, lookup relationship,
master-detail relationship, checkbox, date, date/time, email, number,
percent, phone, picklist, text, text area, or URL field type.

The field_on_lookup_object must be the Record Name field on a custom object.

The field_on_lookup_object and lookup_value must be the same data type.

If more than one record matches, the value from the first record is returned.

The value returned must be on a custom object.

You cannot delete the custom field or custom object referenced in this function.

This example
checks that a billing postal code is valid by looking up the first five
characters of the value in a custom object called Zip_Code__c that
contains a record for every valid zip code in the US. If the zip code is
not found in the Zip_Code__c object or the billing state does not match
the corresponding State_Code__c in the Zip_Code__c object, an error is
displayed.

AND(
LEN(BillingPostalCode) > 0,
OR(BillingCountry = “USA”,
   BillingCountry = “US”),
VLOOKUP(
$ObjectType.Zip_Code__c.Fields.State_Code__c,
$ObjectType.Zip_Code__c.Fields.Name,
LEFT(BillingPostalCode,5))

<> BillingState
)


Cheers!!!