Index Seek and Index Scan are operation for query tuning in execution plans.

Table Scan scans every record of the table. So the cost of proportional is the number of rows of that table.

The Index Scan is preferred only when the table is small.

Index Seek only touches the rows which qualify and the pages that contain that qualifying rows, so the cost of proportional is the number of qualifying rows and pages instead of the number of rows in the table.

Index seek is preferred for highly sensitive queries.