A heap is a table without a clustered index.
so, a table that does not have clustered index is a heap table.
Indexes are used by queries to find data from tables quickly. Indexes are created on tables and views. Index on a table or a view, is very similar to an index that we find in a book. If there is no index to help the query, then the query engine, checks every row in the table from the beginning to the end. This is called as Table Scan. Table scan is bad for performance. Indexes tend to improve the performance as it takes memory slots which are located on the disk. If a table does not have a primary key defined, SQL Server stores its rows in an unordered structure called a heap.
Ideally, we would index columns, that appear in WHERE clause or JOIN condition. An index helps to speed up SELECT queries and WHERE clauses, but it slows down data input, with the UPDATE and the INSERT statements. Indexes can be created or dropped with no effect on the data.
