Filtering Groups:
WHERE clause is used to filter rows before aggregation, where as HAVING clause is used to filter groups after aggregations. The following 2 queries produce the same result.

Filtering rows using WHERE clause, before aggrgations take place:
Select City, SUM(Salary) as TotalSalary
from tblEmployee
Where City = 'London'
group by City

Filtering groups using HAVING clause, after all aggrgations take place:
Select City, SUM(Salary) as TotalSalary
from tblEmployee
group by City
Having City = 'London'

From a performance standpoint, you cannot say that one method is less efficient than the other. Sql server optimizer analyzes each statement and selects an efficient way of executing it. As a best practice, use the syntax that clearly describes the desired result. Try to eliminate rows that 
you wouldn't need, as early as possible.

It is also possible to combine WHERE and HAVING
Select City, SUM(Salary) as TotalSalary
from tblEmployee
Where Gender = 'Male'
group by City
Having City = 'London'

Difference between WHERE and HAVING clause:
1) Apart from SELECT queries, you can use WHERE clause with UPDATE and DELETE clause but HAVING clause can only be used with SELECT query. For example following query, which involve WHERE clause will work but other which uses HAVING clause will not work :

update DEPARTMENT set DEPT_NAME="NewSales" WHERE DEPT_ID=1 ;  // works fine

update DEPARTMENT set DEPT_NAME="NewSales" HAVING DEPT_ID=1 ; // error
Incorrect syntax near the keyword 'HAVING'.: update DEPARTMENT set DEPT_NAME='NewSales' HAVING DEPT_ID=1

2) WHERE clause is used for filtering rows and it applies on each and every row, while HAVING clause is used to filter groups in SQL.

2. WHERE filters rows before aggregation (GROUPING), where as, HAVING filters groups, after the aggregations are performed.

3. Aggregate functions cannot be used in the WHERE clause, unless it is in a sub query contained in a HAVING clause, whereas, aggregate functions can be used in Having clause.
Ref: http://csharp-video-tutorials.blogspot.com/2012/08/group-by-part-11.html