I have used speedtest whenever I need to clear my doubt about my internet speeds. Here, let me comment about the website of speedtest.net.
Well as the name suggest, this website is about checking the speed of data( uplink/downlink) that is provided by your ISP to you machine. Please consider this that, speedtest.net is only done with respect to your machine and it may give you incorrect speed considering a case when multiple users( or machines) are connected with same router. So check the internet load at your place before checking the speed. The same information can be found on their service and support page.
To use this website, you need to install adobe flash player( not for mobile version) or you can use their beta module for speed test( it will give you same results) . Speeds can be output in kbps/mbps depending upon your choice and test results will be save for future based on your ip address. You can choose to change the server also.
Website is pretty userfriendly and does its job, infact it is the best in class. It loads pretty quickly( tested on chrome) and does’nt have any glinches. To use it on mobiles, you need to download its mobile applicatioon, it wont work on mobile browser.