Heap Table: A table which doesn't have a clustered index is called Heap table.
We can describe a 'heap table' let SQL server do a table scan on it while retrieving data, which obviously not good. So try to avoid heap tables in your database to give 100% chances to get the performance.
You can find the heap tables by using following T-SQL statement.
USE [YourDataBaseName]
GO
SELECT SCH.name + '.' + TBL.name AS TableName
FROM sys.tables AS TBL
INNER JOIN sys.schemas AS SCH
ON TBL.schema_id = SCH.schema_id
INNER JOIN sys.indexes AS IDX
ON TBL.object_id = IDX.object_id
AND IDX.type = 0 -- = Heap
ORDER BY TableName
