We can also divide up internal table types into three kinds 
by their access type:

Standard tables. In a standard table, you can access data 
using either the table index or the key. Since the key of a 
standard table always has to be non-unique for 
compatibility reasons, the system searches the whole table 
each time you access it using the key. Consequently, you 
should always use the index to access a standard table 
whenever possible.

Sorted tables. In a sorted table, the system 
automatically stores the entries and inserts new entries 
sorted by the table key. The system uses a binary search on 
the table when you access it using the key. You can specify 
the key of a sorted table as unique. You will often use the 
key to access a sorted table, but it is also possible to 
use the index. Standard tables and sorted tables are 
generically known as index tables.

Hashed tables. You can only access a hashed table using 
the key. There are certain conditions under which you can 
considerably reduce the access times to large tables by 
using a hashed table. The key of a hashed table must always 
be unique.

Internal table are of two type :
1 With Header Line
2 Without Header Line

With Header Line.
there is header attached to the table it use to 
populate as well as retrive data from the internal table.

it is the obselete now

With out header Line.

We use work area..
same funcitions but the name is not same as internal table.

Internal table are of there types:
Standard table:  


Index table: 
                 uses linear search 
                 

Sorted Table
                 user binary search 

Hashed table  :  uses hash key field search.

earlier i have given declaring standard table by mistake.