A functional field is a field whose value is calculated by a function in openerp, hence in openerp functional fields values is calculated by the function and its process of function given below, Parameters:

fnct, 
arg=None, 
fnct_inv=None, 
fnct_search=None, obj=None, method=False, store=True 
fnct_inv_arg=None, 
type=%green%float%black%, 

where
Type is the field type name returned by the function. It can be any field type name except function.
store- If you want to store field in database or not. Default is False.
method whether the field is computed by a method (of an object) or a global function
function is the function or method that will compute the field value. It must have been declared before declaring the functional field.

Example Of Functional Field Suppose we create a contract object which is :

class hr_contract(osv.osv): 
_name = hr.contract 
_description = Contract 
_columns = { 
name : fields.char(Contract Name, size=30, required=True), 
employee_id : fields.many2one(hr.employee, Employee, required=True), 
function : fields.many2one(res.partner.function, Function), 
} 
hr_contract() 
If we want to add a field that retrieves the function of an employee by looking its current contract, we use a functional 
field. The object hr_employee is inherited this way: 
class hr_employee(osv.osv): 
_name = "hr.employee" 
_description = "Employee" 
_inherit = "hr. employee" 
_columns = { contract_ids : fields.one2many(hr.contract, employee_id, Contracts), 
function : fields.function(_get_cur_function_id, type=many2one, obj="res.partner.functio 
method=True, string=Contract Function), 
}