There are five logical/physical operators in SQL Server related to Index scan ,Index Seek and Table scan.
(a) Table Scan
(b) Clustered Index Scan
(c) Clustered Index Seek
(d) Index Scan (Non- Clustered index Seek)
(e) Index Seek (Non- Clustered index Seek)
Table Scan retrieves all rows from the table (if you have no WHERE Conditions). Basically, before returning the rows, it traverse through all data pages related to the table. If you have where condition, though it travel through all the pages only those rows are returned which satisfy the conditions. When you do not have Clustered index on the table it does a table scan. In other words, both Clustered Index Scan (clustered index is nothing but the data itself) and Table Scan are same because in both method system traverse through all the data pages. Generally, you should avoid table scan.
Clustered Index Scan
Clustered Index Scan is nothing but horizontally traversing though the clustered index data pages. Clustered Index Scan return all the rows from the clustered index (Clustered index is nothing but Data). If you have where condition , only the satisfying rows are returned, but system traverse through all the data pages of the clustered index. Both Table scan and Clustered Index Scan are generally considered to be bad. But at times like if the table is small contains only few rows table or clustered index scan may be good also
Clustered Index Seek
Clustered Index Seek traverse vertically right down to the Data page where the requested data is stored. Basically any seek is vertically traversing though the B-Tree structure (as we all know the index is stored in B-tree structure in sql server). System does a Seek when it find a useful index and generally its done for highly selective query.
Index Scan or Non-Clustered Index Scan
As already said, Scan is horizontal traversing of B-Tree data pages. But in this case it horizontally traverse though the Non-Clustered index available. Its not same as Clustered index scan or Table Scan. In SQL Server , while reading execution plan you can find only Index Scan not Non-Clustered Index Scan. But you must read Index Scan as Non-Clustered Index Scan.
Index Seek or Non-Clustered Index Seek
As already mentioned Seek is Vertical traversing of B-Tree to the data page. But in Index seek it vertical traversing of Non-Clustered Index. Generally, its considered as the best option for high selective query.