An index seek shows that the index is used in the table and
index scan shows that index is not used in table

Index Scan is nothing but scanning on the data pages from 
the first page to the last page. If there is an index on a 
table, and if the query is touching a larger amount of 
data, which means the query is retrieving more than 50 
percent or 90 percent of the data, and then optimizer would 
just scan all the data pages to retrieve the data rows. If 
there is no index, then you might see a Table Scan (Index 
Scan) in the execution plan.

Index seeks are generally preferred for the highly 
selective queries. What that means is that the query is 
just requesting a fewer number of rows or just retrieving 
the other 10 (some documents says 15 percent) of the rows 
of the table.


In Index Scan the cost is proportional to the total number 
of rows in the table. Thus, a scan is an efficient strategy 
if the table is small or if most of the rows qualify for 
the predicate.


In Index Seek the cost is proportional to the number of 
qualifying rows and pages rather than to the total number 
of rows in the table.