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 .