can Anyone tell me what is DB connect? what is use of it? how it use? suggest any tutorial link for that.

You can use DBconnect to link your drawing entities to database records or to display database data as attributes in your drawing. Can be quite powerful when used properly.
 
You can find more information in AutoCAD Help.

if i change in database (excel) file then it change in autocad drawing file? like excellink lips file from xanadu. that mean if i chang number of attribut it chang in autocad.prakash_anu2009-06-09 14:45:32

Yes - you can setup it this way (you may need to issue an update/regen command).

I have make a block with attribute with a different value 1,2 & 3 with name of chair in block now I extract data in excel format. then I each chir with dbconnect. Now I want to change the value of chair in excel & it   reflect in drawing. How can I do this? that means how i set up dwg this way?