Datareader is forward and read only recordset in which the 
data manipultions are cant carried. but dataset is the 
miniature of database in which the datas can be accessed 
randomly and data manipulations can be carried

DataReader
Datareader is like a forward only recordset. It fetches one
row at a time so very less Network Cost compare to DataSet
(Fetches all the rows at a time). DataReader is readonly so 
we cannot do any transaction on them. DataReader will be 
the best choice where we need to show the data to the user 
which requires no transaction ie reports. Due to DataReader 
is forward only we cannot fetch the data randomly. .NET 
Dataproviders optimizes the datareaders to handle the huge 
amount of data.

DataSet
DataSet is always a bulky object that requires lot of 
memory space compare to DataReader. We can say the dataset 
as a small database coz it stores the schema and data in 
the application memory area. DataSet fetches all data from 
the datasource at a time to its memory area. So we can 
traverse through the object to get required data like 
qureying database.

The dataset maintains the relationships among the 
datatables inside
it. We can manipulate the realational data as XML using 
dataset.We can do transactions (insert/update/delete) on 
them and finally the modifications can be updated to the 
actual database. This provides impressive flexibility to 
the application but with the cost of memory space. DataSet 
maintains the original data and the modified data 
seperately which requires more memory space. If the amount 
of data in the dataset is huge 
then it will reduce the applications performance 
dramatically.

The following points will improve the performane of a 
dataset...

1. Don't use the commandbuilder to generate the sql 
statements.
Though it reduces the development time the query generated

by the command builder will not be always as required. For 
example
To update the details of an author table the command

Dataset is cache of records retrieve from data source.
Datareader is used to retrieve data from datasource.

1.Using Datareader we can read the data readnly,forwordonly 
basis.
2.Only one record in a memmory at a tme  
3.Faster
4.Doesnot hold any data
5.Data can not be transfer to datasource
Wher as in Dataset
1. we can read and write and allow us to navigate through 
the records forword and backwords.
2.Pulls the records from the datasource and saves in IIS 
memmory.
3.Slower
4.Holds datatables and has the information about
relationship between those tbes.
5.Data can be transfer to datasource

DataReader
Datareader is like a forward only recordset. It fetches one
row at a time so very less Network Cost compare to DataSet
(Fetches all the rows at a time). DataReader is readonly so 
we cannot do any transaction on them. DataReader will be 
the best choice where we need to show the data to the user 
which requires no transaction ie reports. Due to DataReader 
is forward only we cannot fetch the data randomly. .NET 
Dataproviders optimizes the datareaders to handle the huge 
amount of data.

DataSet
DataSet is always a bulky object that requires lot of 
memory space compare to DataReader. We can say the dataset 
as a small database coz it stores the schema and data in 
the application memory area. DataSet fetches all data from 
the datasource at a time to its memory area. So we can 
traverse through the object to get required data like 
qureying database.