Dataset is disconnected method where Datareader treated as 
connected method.

Connected methods gives faster performance where as 
disconnected get low in speed and performence.

connected  can hold the data of single table, where as 
disconnected can hold multiple tables of data.

Disconnected are the forward only and readonly objects 
where as connected or not.

connected data needs connection to be created to access
hence slower while disconnected is in memory data thats
faster access.

to access connected data you use ADO.NET whereas for
disconnected you do not use.

disconnected-data can be accessed from multiple tables in a
dataset.
connected- .net runtime creats an instance of the datatable
to hold data.

in disconnected data you can create a window to the
datatable in the form of a dataview used to sort and filter
data..which cannot be done in connected data access

connected you need to use a read only forward only data
reader,disconnected you cannot.

disconnected data in a dataset can be serialized/deserialized.

Data Reader:-
Data Reader is a read only and forward only data
You can access one table at a time.
Data reader cant persist the data.
Data Reader is connected architecture.
Data Reader is faster than Data Adapter
Data Reader is faster than dataset operation
Data Set:
Data Set is disconnected architecture.
Data Set is persist the data.
You can access more than one table
Data Set is main for storage tool in ADO.Net

Data Reader is a forward only and read only data
DataSet is used to maintain relationships between multiple
tables.

Data Reader can't persist the data
Data Set can persist the data