Workarea contains the multiple records and whenever
headerline contains single record.
Declarations are as 
lt_mara type standard table of mara : workarea
ls_mara type mara  : Headerine

Workarea and headerline both are contain single record 
structure.normally we have two types of internal tables. 
 1. with headerline
 2. with out headerline

in with headerline internal table it self first row 
contains workarea. In case of without headerline internal 
table it will contain work area and body.

Headerline creates default WA for the internal table with 
same name of Internal table.
Both WA & headerline holds single record

internal table with header line  contains header and body. 
any record which is to inserted into the internal table 
body  passes to header line first then it is appended or we 
can say pushed to body of internal table. The header and 
body has the same name. 
Suppose we have a internal table having fields NAME, AGE.
and our internal table name is ITAB.

itab-name = 'Rakesh'.
itab-age = '26'.

Append itab to itab. (" appends headerline ITAB to Body IT")
or we can use :- append itab. (both the above code works 
same).

whereas in workarea it also works similarly as a header 
line.
only difference is that its a headerline which is declared 
explicitly and it is not attached with an internal table .

to append the work area containining a singel record into 
internal table without header line :-

wa-name = 'Rakesh'.
wa-age = '26'.

Append Wa to ITAB.

I suppose this information will clear doubts .

please dont write wrong answer mr. SJHA ..do you know what 
is sap or abap? cartoon


function of work area and header line is same.
Both are used to processed the data from internal table .
as i know the default work area of an internal table is a 
header line.