The Clustered Index Seek operator uses the structure of a clustered index to efficiently find either single rows (singleton seek) or specific subsets of rows (range seek). Because a clustered index always contains all columns in a table, a Clustered Index Seek is one of the most efficient ways for SQL Server to find single rows or small ranges, provided there is a filter that can be used efficiently.
The behavior of the Clustered Index Seek operator is in fact exactly the same as the behavior of the Index Seek operator, with only a very few differences as noted below. Though these two operators do have different names, not only in the graphical execution plan but also in the underlying XML, I suspect that in reality they are both using the same internal logic and not a copy of it.
Read the whole thing.