internal table is a temporary storage area that exists only 
at program runtime.

Types
1.standard.
   The key access to the standard table is always linear 
search.It has non unique key.Append , insert and collect 
work.It has linear index operation.
2.sorted.
   This table is always internally sorted.
append with improper order can produce arror.use either 
unique or non-unique key. always use binary search.

3.hashed.
It has unique key.no index.always use hash algorithm.
append and collect will not work.