Some basic and important Execution Plan rules

• Read from Right To Left and Top to Bottom.
• Read it from right to left, as data flow comes from right to left side.
• If you see nested loop, the upper side is the outer query, the lower side is inner query, inner query is executed once for each outer query.
• Follow Arrows until next branch from below.
• The thickness of the arrow reflects the amount of data being passed.
• execution cost of inner side for nested loop is per execution of outer side.
• If you see hash join, it means that hash table is built for upper (outer) query, then for each rows in lower (inner) query, query processor will look into the hash table to find match value.
• The number that displayed as a precentage is the relative cost to the query for the operator.

• Table Scan - scan all the rows of the table in order to select the results set.
• Index Scan - scan all the rows of the indexed table in order to select the results set. We will scan indexed table when all of the table rows, or the most of them will be returned.
• Index Scan - Seek the table, using indexes, in order to select the results set.
• In the most of the cases, seek is better than scan.

http://msdn.microsoft.com/en-us/library/ms175913.aspx

http://www.simple-talk.com/sql/performance/graphical-execution-plans-for-simple-sql-queries/