BDC Data is a structure with 5 fields.
1.program  char  holds program name responsible for the scrn
2.Dynpro   numc   screen number
3.DYnBegin char  'x' indicates start of the screen.
4.Fnam     char   Field name
5.Fval     char(80) Field value

BDCDATA tables name

PROGRAM----------> Prgram name
DYNPRO-----------> Screen no
DYNBEGIN---------> Start of screen
FNAM-------------> Field name
FVAL-------------> Field value

total 5 fields.

the fields of the bdc program is given below.

program :name of the module pool programm.
dynpro  :screen number
dynbegin:it indicates for the new screen
fnam    :name of the fields in database
fval    :fields values are assinged with the fields

bdc data is having 5 field.

You can these information about any field by 
just clicking the particular field
then Pressing F1 and after that clicking 'Techincal propery'



program name : name of the program.
dynpro:  dyn = screen , pro = number.
          every screen in sap having a specific dynpro.   
dynbegin: 'X' shows the new screen
fname: name of the specific field in the screen
       every field is having different field name
fval: value assigned to the field name .