ab is a tool for benchmarking web servers. It is designed to give you an impression of how your web server installation performs. This especially shows you how many requests per second your web server is capable of serving.
To benchmark a web site, use ab command provided by Apache.
ab -c 200 -n 15000 http://site-to-benchmark-here.com/
This will start 15000 requests to the server specifified. 200 requests at a time.